I don't think they "sell" animals like a pet store perse, but they do have rescue orgs. there trying to raise money, awareness, and to give the public the chance to see available pets for adoption. The exotics were probably there just for demo. The expo used to be in Ft. Washington, which would of been super convenient for me since its less than a mile away. Then when that facility closed it moved out to Reading. I'm pretty sure its an annual thing, depending on facility availability. I've never been before, but I'm expecting it to be a big trade show for the pet industry. The main events, speakers, activities, are usually held on the busy days, which in this case would be Saturday. So they can guarantee lots of foot traffic to offset the cost of the extra attraction/activity. Another tip with trade show vendors is if you go on Sunday when its near closing time, and they are packing up they will usually cut you a good deal on products.
