I would go with a glass aquarium. Put frozen ice packs (the plastic blue kind, you know?) in, or under the cage. Under is better so they can't chew on it. This will give them the option of cooling off by laying on one.
Also a battery powered fan, would be useful.

I would suggest NOT selling them at the flea market. People might buy them and walk around with them for hours in the heat, and that's just cruel.

Advertise on your local craigslist, and other sites like that, and have people come to your home for them. I don't think you'll sell enough at the flea market to make it worth it for you, or the animals.