Unless the stamps are otherwise labeled somehow, I would prefer to buy them still on the pages. I'll most likely remove and remount them, but having them still on the page makes it easier for me to find the corresponding pages & spaces in my own albums. If they're just loose in glassine envelopes and not labeled, it can take me a lot longer to find the right place to put them, particularly on some issues that look alike. If the glassines or cards are labeled with catalog numbers, that would actually be better than having them on pages. Either way is fine with me, as long as they're organized somehow.
The other thing is, if you want to sell them on
ebay, taking pictures of album pages is probably easier than photographing or scanning in stamps individually. Buyers on
ebay are used to seeing collections offered this way as well, so I don't think there's a lot of downside to leaving them on the pages.
Edited to add: it also depends somewhat one the appearance of the pages. Are the stamps mounted neatly and do the pages look well-arranged and attractive? Or is the page crammed with stamps that appear to be mounted haphazardly? Presentation is a key aspect of selling. If you have an "adult" album (for lack of a better term) made by a major manufacturer such as Scott, Lighthouse, Lindner, etc, chances are the pages look nice and the stamps can be sold on the pages. If it's more of a beginner album (such as made by H.E. Harris), the stamps tend to be more crowded and the pages (and therefore the stamps) may not look as attractive to serious collectors. In a nutshell, if they look good on the page, leave them there. If not, removing them is probably a good idea.