When you buy off eBay, pay attention to the seller description and feedback. Does he sound like he knows what he is talking about? Provide good pics, describes all indicators, meters, channels are working, that he will pack well, etc.
Some Realistics can be very reasonably priced. I like the STA-90 at 45 watts and the STA-77 at 16 watts. Make sure the pointer turns red when capturing stereo. If not mentioned, it's a good question to ask the seller. Not just to get an answer but to see how fast he responds and how well he communicates.