Any car is a time bomb. No matter what you do or how well you maintain it.
Way way back in my mechanic days, I replaced many motors under warranty that had 30k or less. Mopar and GM. You just never know when that one crucial piece will fail bringing the whole thing down.
I'd say be upfront about any little thing you can think of. I've sold and swapped many a piece of shit with my buddies and we're still cool. But they all knew everything about the cars/trucks/etc.
I did have one instance with a 02 Sonoma though. I got it, never had the chance to look it over, traded it to a friend for a running 1 ton with a 454 I wanted. That little truck was beyond fucked. I can't even describe the things that were hacked in and chopped off underneath. It had been bagged at some point, the bottom of the frame was paper thin where it had any frame left, the fuel pump wiring harness had about 6 extra feet spliced in with household wire nuts, it was beyond repair. I ended up feeling so bad that I bought it back and took it to the scrap yard.