I'll state here that I am not published, but I've been to a BUNCH of conventions.
I would think it would honestly depend on the convention.
I've been to GenCon, and I've been to World Fantasy. Quite honestly, I did not buy books at either conference.
GenCon doesn't really have a specific bookstore that I saw when I was there. It's a great con, but book sales were happening by the individual authors who had brought their own copies, unless their publisher was at the con...and most weren't. There were a few people with media tie-in books who were published by Wizards of the Coast, or other similar gaming companies, who had their books at the company's tables. There were some independent bookstores who had booths and books, but I didn't see a relationship between that and the authors present. (I will say that I highly recommend this con and their writing symposium, but as a READER I did not see a lot of opportunities for bookselling. Please keep in mind that I was there three years ago...there may have been some changes. I did spend a lot of time there with a friend who was an author signing in Author's Alley, so please also consider that, too.) It's my understanding that either last year or the year before, some of the authors at that con may have set up some kind of group signing event with a local bookstore, but I don't know if it was at the store or at the con itself. You might want to look for the list of authors who were present and send them an email if you can, asking for their impressions.)
I went to World Fantasy last year, and I saw some selling books in the dealer's room, but I don't know what relationship they had with people actually doing readings. People at the con were talking more about the books in the freebie bags and things that weren't out yet than the books being sold in the dealer's room. There was a group signing event, but I did not attend it; I had an opportunity to do some networking instead, so I don't know how those sales were handled.
Of course, an actual author who sold books at these conferences might have a different take, but these were my impressions. I had great things to say about both conferences and highly recommend them both as an attendee, as an aspiring writer and as a reader, but I didn't really see visible, connected sales going on.
I'm not sure that I would necessarily look at a conference as a place to sell books, but rather as a place to network, talk writing, make contacts, and meet readers. Selling would be icing on that cake, but like I said, I'm not at that point yet.
If I'm reading this wrong, I hope someone jumps in. I know that some of the authors from the Gen Con Writers Symposium are on these boards....I hope they still are...