The TPS is sometimes damaged, although it's usually the plug that's damaged and not the sensor itself (they can sometimes be salvaged). Most of the time you buy an engine it's in good condition. If a lot of people buy an engine from somewhere and they all suck, that place usually won't stay in business long. That said, sometimes a lemon does sneak through.

Some good advice about an a used engine, assume the previous owner did nothing. Don't assume the belts and hoses are in good condition, be willing to replace anything that might need it. A good freshing up with new plugs, wires, cap and rotor and a new fuel filter is also a good idea. Buy quality filters and fluids too. Seeya.