Oh, I think people DO go that route fairly often. But I also think most of them don't understand the consequences till too late.

Years and years ago, when I was working as a repair plumber, Lowes began to sell water heaters at $5 - 10 more than they cost me buying them "wholesale". At the time, I was charging around $150 to swap out an electric heater, and adding about $60 to what I paid for the heater. More than a few people thought it would be a good idea to save that $50, and buy it themselves.

Initially, I got P.O.d. But I realized that it was a reasonable choice for them to make . . . based on the limited info they had. So I explained that about 5% of the heaters I installed had some sort of problem within the first month -- and that I'd correct that for free, if I sold the heater, but that it would be another service call if I had installed a heater from Lowes. And, I pointed out that maybe 2% of the heaters had a MAJOR problem within the warranty period, requiring replacement, and that (again) that replacement was free, if I sold the heater, but another $150 if they bought the heater.

Pretty much everyone decided -- once they were informed -- that saving that $50 wasn't such a good idea after all. But, they wouldn't have known if I'd just gotten mad, and not explained calmly.

When you're doing the work yourself, it makes sense to buy from the best source, so long as they'll honor basic equipment warranties. But hiring a contractor to install YOUR materials often leads to a real mess when something goes wrong.