Other than eBay

I think that Ebay is okay. There are For Sale and Wanted to Buy forums here on the AppleFritter. If you get specific as to what you want and how much you want to pay for it, you might find some luck here. The best deals are on the rare thrift store finds. Craigslist is a remote possibility. No matter what you find, if you can find a seller in your area from whom you pick up the unit and avoid the shipping costs, that's how you save a lot of money.

As stated, Craigslist (I've bought many a machine both Apple II and Mac from Craigslist) as well as thrift stores and technology recycle stores.
Also, there is quite a lively trade/buy/sell going on at the Apple II Enthusiasts FaceBook group. Many of us have excess machines/parts/etc. and will often see if someone from the group is looking for something (or a newcomer looking to get into the obsession...I mean hobby!) and you can often find some good deals. Just ask!
There are a few buy/sell groups on facebook too specifically for retro tech but they don't often move many items. Usually people looking for ebay prices on a facebook group...doesn't typically work.

Apple II Enthusiasts on Facebook

Only other place is this one:

http://www.blujay.com/index.html?page=profile&profile_username=drkenb&