One good place is the group SewItsForSale on Yahoo. It's best to create a Yahoo ID! (free), after which you can join that group. They do have rules, so be sure to review them before posting. Here is a link:
https://groups.yahoo.com/neo/groups/SewItsForSale/info
The Quilting Board here has a section for selling, but it also has rules including how many posts you need to have made on the forums before you can submit an ad. Here is a link to that section:
http://www.quiltingboard.com/items-sale-trade-f20/
Be sure to read the "sticky" for the rules.