Choose the right seller. I don't like most commercial sellers, neither camera shops (who have too high reserve prices) nor vendors who sell cameras without understanding them (they will preface their ad with lines such as "I don't know how this camera works, but I know what cameras of this brand sell for ..."). Avoid them. Buy from a private seller who/is was also a user. Look for lines such as "I have used this camera for the last 5 years, until last months, when I bought ...". Such sentences tell you that the equipment actually works, and that the seller cared for it.
A private seller will typically have sold a few items on eBay (somewhere between 0 and maybe 20 to 50). Anyone with hundreds of sales on eBay cannot be considered a private user/seller.
Choose the right item. This does not mean the right model, but the right unit of the particular model. Again, look for the seller's description. Avoid items described as "everything seems to work right", as it translates into "I have no clue whether it works." Avoid also "I have not tested this unit". This usually means the seller has tested the equipment and it does not work (but he/she cannot say so without losing money). Instead look for statements such as "everything has been tested and works spot-on", or "I have used this equipment until yesterday as my primary equipment. All functions work 100%."
When buying, know how much you value cosmetics vs. function. Collectors value cosmetics highly. "Users" care more for functional integrity. Working cameras that look ugly are therefore often labelled as "users".
(c) Christian Wagner, 2001