Amibay.com. You'll mostly be buying from Europe, not sure where you're located. Prices aren't much better than on eBay, however.
I've noticed some eBay mentality has infected AmiBay. Listings go up with ridiculous prices, or at least prices which do not please the market, the listing stays up for months, an inquiry comes into the thread about the price and the seller stays firm -- nicely or otherwise.
Tells me they're not serious about selling the item and just wasting the time of all involved.
Join the Commodore Amiga Facebook page!
I realize like AOL back-in-the-day, Facebook = Internet for a large number of people, but there are still a good number of people not on Facebook. If you are then joining will be a useful thing. The rest of us have other options.
For instance, I trawl Craigslist every so often with the simple search
Amiga site:*.craigslist.org
I have netted a few good deals for Amiga, TI, Commodore, Atari, Sega, and Nintendo gear this way, as well as a fairly difficult-to-find SonyEricsson mobile phone. (Great, I just gave away my secret.)