Ebay is a very fickle place. You never know what people will be willing to pay. Sometimes I'll see an item I want go for $40, sometimes the same item will be $250. I guess it depends on who is checking Ebay at the time and how wealthy they are.
Judging prices is very tough too. I had a near complete collection of Fushigi Yuugi pencil boards, which I started at $15, but ended up selling for $300. Insane. People go kinda nutty with Ebay, especially those last minute bidders. Having an auction which ends on a weekend is best, I think.
Getting your feedback up is really important- buyers are much happier dealing with sellers who have high positive ratings (and vice versa). There are *lots* of bums who either don't send an item or don't send payment. It sucks, but that is just how it is.