So, really, I suppose that site is only good if you have quite a bit of extra money to throw around, but it is the one I found. eBay is often a better (and cheaper) alternative, though.
If you really want old Avalon Hill games and other such things, you could look here: http://gamersalliance.com/. However, you won't find the good stuff cheap. Also, there is a membership fee required to get access to the catalog. They have Avalon Hill, TSR, and others, depending on their supplies at the time.