I've bought a lot of stuff off of ebay. It's usually something that's not available in stores anymore or is just a weird little thing that I wouldn't want to drive around looking for. When I first got my computer I went and bought a bunch of old G.I. Joe figures to replace the ones I had as a kid.
I've only made one sale on ebay, a couple Transformers figures I had kept in their boxes. I made about $150 profit on that, so I'd say it was worth it. I charged a flat rate of $15 for the shipping which was pretty fair.