I use @Scott's approach by making inquiry.

Hopefully I don't have to ask. "Do you keep cattle in that closed bedroom?"

'Why do you ask?" they respond.

"The fragrances present are not compatible with the ambiance of a house at the value you want."

"What? You smell something?" responds the nose blind home owner.

"Yes I do. To be honest with you, these are not tears of joy."

Rarely does the owner FIX oder issues even when they can be led out of the house on a leash. 

One story of great success involved odor that drafted into the house from the attached garage where all the 'chemicals' were stashed. Moved the chemicals to the outside shed. value went up $15,000.

Owner actually said later..."That was what was making my eyes water. I blamed the oak pollen from the back yard tree!"  I was afraid they were going to close with, "Now we don't need to move."

I would not wait and let them hear it as feedback after it has gone on the market, because it needs to be dealt with long before it goes on the market. I also consider it my duty to them to be truthful, although as tactfully as possible.  

You've received some good answers here... and I would suggest going in with a solution, not just a statement.  There are some good air deodorizers units on the market.  Best to get rid of the odor than mask it.

Tony Whiteside you can tell your client that it's those potential buyers who feel that way!

The way the home should 'smell' is what THEY want - and not what the sellers have.

Tony Whiteside Be honest with the seller. If you aren't a buyer will.

I would include that information when providing feedback from showings. 

Just be upfront with your clients. It's better to get the smell and other problems resolved rather than driving buyers away.

I guess the ol' clothespin on the nostrils is out so my best answer would be, . . . . . delicately.

This Q has been posed before and answered by professionals, and all I can say now is I need an aspirin. LOL!
There is a saying among investors: "If the house smells, it smells like MONEY."

Translation: No one's gonna pay your asking -  and you'll end up letting us steal it.
This is one of the most important things a homeseller who wants a closing and a decent net needs to know.

Explain that there are smells that are normal and natural to the people living in the property, but they are not neutral to newcomers to the property.

You have to put your business hat on when you sell a property and that's what you should tell the sellers.

Pinch your nose firmly, and collapse on the floor gasping for air??!! 

I use as much tact as I can muster but in the end some people don't get it and I have to use stronger words like your house has a pet odor or cigarette odor, and it will offend buyers and hurt the sale of your home, etc. 

I come right out and tell them nicely

I would just be honest.

Tony I have no sense of smell.. and probably am lucky that I cannot smell anything.. but I can understand that certain smells are offensive to some people.

One good way is to report it as feedback from showings.

Mr. & Mrs. Seller, feedback has said that your home has a foul odor....can we do something about this and get the house sold ...

After previewing a home if there are bad smells I ask "do you have cats?" If they say yes I just say "I thought so". They usually get the message. If not press on!

Sometimes it pays to be to the point and blunt.

What is that aroma I smell?

This is a good topic for discussion after they have signed the listing agreement and you are telling then what needs to be done.

You have to know how your seller will react to know what type of words to use, you have to be honest, but soft and include a solution not just state their is a problem. Many times it is the buyer comments that saw the home that state it over and over that get the point across.

There is not really an easy way to tell them, but you just have to be honest and let them know.  Otherwise, they will be looking at you and blaming you for their house not selling.  I usually give them a list of everything they can do to get their home sold quickly-this would be one of the items on the list.

Good morning Tony. Never an easy answer, but the only way I know to handle situations like this is to be complete forthright, but it is not what you say, it is how you say what you say.

I think the direct and tactful approach is best.  "excuse me, but I noticed an smell / odor as I walked in.....

     I listed a vacant townhouse recently.  It smells like cigarettes.  The owner and I stood in the kitchen discussing the Listing Agreement, and he took out a pack of cigarettes, and lit one!  

    It was time for a "Come to Jesus meeting".  He grumbled, but he agreed to step out into the back yard to finish the smoke.

Just tell them they need to do something about the (Smoke,pet,whatever) oder in their home.  The Buyers will notice it.

If there's a home staging consult prior to listing the issue will be dealt with.  Must be direct and honest. Source of smell removed not covered up.

In my experience, when cats are involved, you have to wash the walls at least 3 feet high.  They spray their territory, so changing the carpets won't get the whole problem.  Washing is effective and easily done, if you don't have wallpaper.  

I think you got some great answers here!

I was just in this type of situation on Wednesday Tony Whiteside - Previewed a home that smelled.  I didn't know if it was animal urine or a food smell - HA!  Other realtors told me it was a food smell . . .  for that reason I did not give it 'high marks' in the Odor Category.  So, the realtor who represens the Seller would be the one to 'share' Home Preview Comments to the Seller.  Now, it sounds like this house has 'odor' year-round, or is this a combination of different homes?  Anyhow, I would ask them about it, because if they live in it daily, they are probably no longer 'smelling' any odor.  Hopefully, no critters passed away somewhere . . .

"Your home smells"

Clothespin on the nose? 

Surprisingly, many homeowners do not feel the odor of their houses. I know exactly what you mean, just be patient and delicately tell them ''the news''. I hope, it is not that bad...:) 

They need to check if they have a mold issue.  If so the smell is the least of their problems.

Tony, I try to have a disinterested third party make the comment essentially that "Your home stinks!" Once that happens there will be discussion whether good, bad or indifferent!

What is that odor?

Do you cook with exotic spices?

I love the smell of leftover good food

Not everyone can enjoy this type of scent

We need to pick a scent that is Universal

Do you do your own cooking? I can smell all your hard work

Some old clients had 2 cats that must have used the entire house for a litter box because the smell was the absolutely worst I have ever smelled. They told me they were replacing the carpet and I jumped in and told me they needed to Killz the floor. After the carpet was put down, it still had a slight odor, and I told them so. An agent commented on it, too. Thankfully it went under contract before I told them to have the carpet pulled up again and floor painted. (They kept their cats somewhere else while it was on the market!)

Tell them they may be used to the smells and don't smell them anymore.  But people who are new can smell them and it is costing them thousands of dollars in sales.

Ask: "Do you have pets?"

And hope they say yes..

Tony,
That is one of the hardest of all. I try to bring in some disinterested 3^rd parties and then pass along what they smelled. Best done by email not in person or by phone.

You have to be honest, buyers will be honest.     If they have pets, they may not notice, and I usually like to say this when I come in.   They then know it was soon as I walked in the door.

Tell them up front that "we" may have a situation that needs attention!

Succinctly and before the home hits the market.  

 Walk in with a can of air freshener. I have been in some real stinky houses. They do sell – eventually.

"sniff, sniff, ... Whats that smell?"

I would suggest they try some plug in type air fresheners adding that we need to have a nostalgic smell to make buyers feel at home.

I just ask them " Did someone die in here"? Then explain that we'll need to work on neutralizing odors to help keep potential buyers looking at the house longer.

"The buyer liked the home, but didn't like the smell."

In person.

Tony,

I think this question has been answered really well.  A