"I've always sold things on here for 50-60% of what I bought them for (after using them for a bit or a while) and that always seemed reasonable to me."
+1 In those cases, I consider the 50% I lost a rental fee.
"Sometimes it takes a bit of bumping and fortuitous timing, as FS items tend to sink fast in the forum and the search function isn't the greatest. Sometimes people who want an item just don't see it as it sinks past. Keep at it."
Timing, IMO, is particularly important. Not a good idea, for example, to post a FS on Saturday evening.