It depends on what you're buying, and if they even know what the heck they're talking about...
If you're buying COMPUTER parts, I highly recommend the FS/FT forum here on anand. Everybody knows their stuff, and you can ask around.
Also, for certain parts, you will want a new one. Mostly parts that may have taken a lot of abuse, 'cause you're not sure what the previous owner did with them.