For exposure to a large potential buying base of ammo buyers, there is currently nowhere better than Gunbroker. Their search engine however, leaves much to be desired since they changed it in recent years. It automatically includes descriptions in search results which floods searches with irrelevant results. They claim to have a relevance filter which puts the most relevant returns near the tp, but is garbage. So be careful with what choice words you use in the title and such, so as to get whatever relevant audience you are after.