Personally all I've ever sold cosplay-wise is wigs, but I'd say you'd have a fair chance at selling your cosplays if you do get a table. I've seen other people sell theirs in the past with decent success, at least based on forum posts, as there's always people who either don't have the time, money, or energy to make their own costumes, so buying used costumes is pretty ideal so long as they're in good or fixable condition.
As for fair pricing, I think that depends on several different factors:
- Did you make it yourself, did you buy it premade, or did you commission someone else to make it?
- If you made it, how much time and money did you spend on it?
- If you bought/commissioned it, how much did you spend?
- What condition is it in? Are there stains or tears?
- Is it a full costume? Are you including a wig or accessories?
Also, if you do decide to sell them, include whatever measurements potential buyers may find important, as they will probably not be able to try them on during the event (the only exceptions to this I can think of are things like jackets or cloaks, which can be quickly and easily thrown on over most summer clothing).