Why would someone go to one of these to buy greater-than-ebay prices to probably buy something all-sales-final?
Some people like the benefit of being able to fully inspect the condition of something instead of an online listing. Some people have a budget to buy stuff like this in person for a con outing.
Personally, I never got the draw. Every time I go to an anime convention I see all sorts of overpriced stuff and I groan watching the people spend $20 on something than costs like $5 or less if you know where to look. If I see something I like at a con, I normally take a picture and research/buy it later when I get home.