Posting a product works almost the same way as with a product on the Tindie Marketplace. Here is an article to help you out: https://tindie.zendesk.com/hc/en-us/articles/204270567-How-do-I-list-a-product-
When you are selecting the category, be sure to select the Flea Market options. There are several sub-categories to help sort in the right place and make it easy for buyers to find exactly what they want!