Randy at Freeminds is a good source, too.
I have a couple hundred books. My favorites are the full set of Studies in the winged globe edition, the Photo-drama of the Creation, amd the Daily Manna (with a preprinted entry for Russell's birthday). I also have a copy of Comfort for the Jews - had to watch e-bay pretty closely for that one.
The prices do seem to be pretty stable. I find the best bargains are when the seller neglects to put 'Watchtower' in the description. I think a lot of people miss those in their searches and the closing prices are consiferably lower.