I have had DOAs which the sellers have refunded, but if something is working when you get it (and for a few days after - always leave 2-3 days of running before leaving feedback!) then dies, I'm afraid you are stuck with it...you pays your money etc.
Generally, if amps are working when you get them, then they usually stay working, unless a fuse or something similar goes, although most will slowly degrade over time (but this won't stop them working completely) due to the capacitors aging.
CD players are completely different as they have a multitude of moving parts. This is one area where I would definitely go for new if at all possible - even sticking with an iPod for now.