I don't know if this is a custom/homebrew site or not, but most commercial "bulletin board" or "forum" type website software have categories built in, so you could have a section for general chip-related topics, another for postcards, another for hot rods, another for sales (and sub-categories for chips, postcards, ashtrays, matchbooks, etc). Rather than paying a commission or fee per listing or sale (which would be a pain to track and a bigger pain to attempt to collect on), charge a small annual fee to users who want to be allowed to post there, and update the permissions accordingly. Problem solved. Sales sequestered. Everyone else isn't bothered. And financially worthwhile to the operators.