A new main is a pricey thing.  Good thing you're doing that before a sale and can enjoy it.  Copper repipe is great, pex is super easy to work with.  There is some concern over rats chewing through it in attic spaces though, so be careful.  Our plumber has seen it happen.

Now with that all said, as someone who works with flippers who've sunked solid 5 figures into plumbing, roofs, rewiring the house electric (knob & tube anyone?) and other "behind the scenes" stuff, we typically get zero ROI for that.  That's one of those things that is expected to be there and working. 

While having new pipes is a great selling point, I think you're over-estimating their re-sale value. People put more value on the pretty, not the practical...as someone selling a 1926  house right now with updated plumbing I can assure you of this.

Most of the homes we flip start with replacement of most of the plumbing with Pex.

Makes total sense to me depending on the age of the home.  We need a new main shut off valve....on the list.

Sounds like a win/win to me Richie Alan Naggar 

I put in plex but did not like the taste of the water, so, repiped the house with copper. Water tastes better.

Great selling point, but not sure about the rate of return!

I think it is a great selling point, but bigger plus is the peace of mind that your plumbing is in great shape for years to come. I have replaced a small section with PEX, but only because I had a drip in my copper that was getting worse. I have plenty of old copper and there are calcification spots that are bound to get worse. If I had your advantage of a retired plumber, I’d take advantage of it

Richie, I do agree with Thomas J. Nelson, REALTOR ® e-Pro CRS RCS-D Vets given your region of the country. 

In our freezing climate unlike yours, PEX is huge. 

It took off like gangbusters during the recession when busted pipes were common in winter foreclosures.

I think it's a good idea to plan ahead. Depending on your houses age, you really should not prolong those updates. In the end, you'll have less headaches.

Everyone that's done a repipe has 'had' to do it.

I think it is good for resale.

We went with copper when replacing polybutylene piping. Not taking a chance a ways down the road with something that may not last and become compromised. The best to you Richie!