I check eBay all the time and have bought a lot of Mosaics at good prices on there the past few years. Of course there are sellers on there who post crazy prices (sights4sound) but one can find bargains if one is patient, knows the value of an item, and knows what one is willing to pay.
For example, a few minutes ago I bought the Mosaic Stanley Turrentine box set for $75. It was just listed an hour or two ago. That is a title that shows up on eBay for an average of around $125. It is a set I wanted to get but, because I had prioritized getting other Mosaics before that one, I decided I did not want to pay $125 or more for it at present and told myself I'd get it when I found a good deal on it, which I just did.
It is a matter of constantly checking eBay, being patient, and sticking to your target price because, with a few exceptions, eventually you will find the Mosaic you are looking for within your price range on eBay..