Due to exceedingly limited space, I have very little beyond what I really need, but on the rare occasion I do have excess to sell/give away, I now follow the following guidelines:
1. 100% satisfaction guaranteed or I'll give you a full refund including shipping. My reputation is not for sale at any price and if you're not happy, I'm not happy. But this is usually a moot point due to 2.
2. If I have something relatively inexpensive, I'll give it away including shipping it on my own dime (like a M.C. cocktail tracball I gave away a couple months ago). It is hard to have a complaint about something you got for free and the goodwill is worth more than a couple bucks IMO.
3. When I do sell something, error on the side of caution when giving a description. A few weeks ago there was an issue with a board pulled years ago and advertised as working. If I pull something working but have no way to currently test, it is listed as untested.
Like I said though, this rarely is an issue because I rearely have anything on hand beyond what I really need...