Here is a decent place to post, plenty of active users. Other options include various facebook groups(not marketplace, it is against their rules), reef2reef, or offerup/craigslist(also against their rules, but many do it). Use an orange filter, get some nice snapshots and give a fair price. It's a relatively straight forward process, but due to corals being animals, finding platforms that allow their sale is a little bit harder sometimes. People also gain a following through instagram which is good if you're willing to ship.