If the price works for you and the owner seems to be honest, go for it. Just go into this knowing that you may have some repair work and definitely will have some maintenance work.
The roof membranes are warrantied for 12 yrs. but not any openings or sealing areas. Those are the spots that leak. Do a very thorough roof inspection and if 90% seems OK, then buy. When you have the trailer, climb up there and seal everything. In fact, I wouldn't buy a used TT without a pressurized leak test. If it did result in some minor leaks, that wouldn't necessarily be a deal breaker but I'd want to know how many there are. I would bet that there will be some.
There will be other issues that you'll have to deal with but a 2 yr. old rig should have some of the bugs out of it. Good luck with your new TT!