Check for local aquarium societies. They typically have auctions at some of their meetings, and it can be a great place to get really cheap used equipment and you can often find unusual fish and plants.
I've bought and sold on it. Lots of good sellers, only one bad deal so far (the guy I bought my flounder from, but that was an off auction side deal.) It's a lot like eBay, just research the seller through feedback and the forums and you can avoid the bad deals. I love Aquabid, I've found some neat fish on the site.