If you don't want to post a product in auction format, the best place to go is
http://music.recycler.com/ The Recycler is a want-ad newspaper from California that, like ebay, has nearly every type of product you can think of. However, since it's based around the L.A. scene there is tons and tons of music hardware that is sold through that little newspaper. Since they came on the web a couple years ago, their user-base has expanded across the country. It's quite easy and free to post an ad, and it's not a task as setting up an ebay page.
In addition, there's another auction site called Digibid.com. I don't really recommend it because they use a storefront-type setup unlike ebay, and it never quite has enough information. There are a fair amount of people who check out that site, but not nearly as many as for ebay or the recycler.