Like buying any used computer with processors hot enough to require active cooling systems, buying a used PS3 or Xbox 360 is a bit like playing Russian Roulette.
The only reason to buy a used PS3 is if someone prioritizes PS2 BC over reliability and lower power/heat, which is not a very good one IMO. If I played PS2 games that heavily, I'd just use a PS2 instead.
Buy a new Slim; get the one year warranty.