I'm thinking of adding a sales/trade area to my forum that I envision operating like Craig's list, where members can post items they want to sell or trade. It would be using the GoodForNothing Classified addon to implement.
Does anyone have any advice on pitfalls I might run into with having a section like this on my forum?
You may want to make some requirements for members who are allowed to sell, for example by requiring that they be a member for a length of time or meet a requirement for postcount or likes.
Think about what type of people will be selling stuff. You may be thinking of this as a place for members to sell used items, but others may see it as a place to sell their shop merchandise, or to sell customized items they've made, or to scalp items, or to offer some kind of service.
Our B/S/T section is very popular and is a big draw for our site. However, it's also a lot of trouble. Scamming is rare, but it does happen. It helps to have a rule requiring a photo of the actual item – not just a promo pic – and some forums require a handwritten note with the member's username.