Ideas on selling Thousand Trails membership

There are a lot of platforms out there you can use for selling just about anything.  You should seriously consider Craig's List.  It is still viable and has a lot of users and a lot of person-to-person transactions occur there.  I believe you can list something like this on eBay, but you'll have eBay's commission based on the final value of the listing.  There's always Facebook Marketplace, which, if you are members of any of the various Facebook camping groups, will get you some visibility in those groups after you put the posting on Marketplace.

More than anything else, I would recommend having all your ducks in a row with Thousand Trails in terms of how you transfer the membership.  I wouldn't put anything up for sale until you have all the paperwork ready to make the transfer happen.