A Corsair 750w should have no problems with a single 480 unless it is very old. You will want to ask about the PSU usage statistics, though.
For every year of average use, a PSU (any kind) can lose as much as 10% of its ability to power things, so that could mean 75w gone in year 1, another 75w gone in year 2, and so on.
Most of the time PSU needs are overestimated and people often buy PSUs too big for their needs. This doesn't hurt, though, because PSUs work best and longest at about 50% load. Ideally, operation would vary between about 40% and 60% load. When people buy more than they need, however, it does help to delay PSU problems. It takes many years of a 10% loss each year for a 750 not to be able to put out 400w anymore, for instance.
The PSU sounds new'ish, like someone else said before maybe someone can't pay their rent or something. I doubt the PSU is that old so there should be nothing to worry about.
I use furmark to test my GPU, and I use 3dMark as well. There are tons of different things people do to benchmark things, though. Some people even zip directories of like 1 gig and time how long it takes and compare that with other computers. Others play the same game on both PCs and see the difference in FPS.
Really the best benchmarking you can do is just to do whatever it is you want to do on the PC and see how it works out. If you want to play BF3 or Skyrim after you buy it then take it with you and load it up and see how it plays before you buy it.
I highly doubt the owner of the PC would forbid you from doing this if he think he will get his $500. Call him up on the phone before you go over there and bring it up before you go if you are worried about it.
HW Monitor is good for testing temperatures if that is your thing.
In any event, temperature problems are usually blown out of proportion especially with no overclocking. They also tend not to be the most expensive things to fix too.
Not buying it because of something you can fix cheaply would be a mistake, as I mentioned before.
In this world, people usually overvalue looks and undervalue performance. Read "Moneyball" if you don't believe me (or even if you do, it is a great book). The best deals are always on things that look bad. If a box is open at a store even if the stuff in it was never touched you can often get 30 - 70% off for that thing. I love buying stuff like that. Haven't got burned yet.