From what everyone says on here sounds like its a big no no.
Why exactly?
I would imagine it's the fact that he sold it and had to buy it back. He's out of pocket and therefore may have cut corners on the repairs.
Tbh looking at it wouldn't cost anything other than a bit of your time. You're under no obligation to buy, and it gives you a comparable for others you may look at.
I'd take a mate though for a second opinion and go with your instinct. At the end of the day it's a second hand car and as we all know - a second hand car comes with second hand problems. It's just that some problems are bigger and more expensive to fix than others!