Craigslist, rvtrader.com, and any others in your area. Price it how you want, and negotiate, or not. Many on here say to never pay asking price, but if it is fairly priced, why go lower?
Put lots of pics in the ad, and state why you are selling it.
Up to you if you take a check, or not, but I insisted on cash. Show the trailer on the street, then go to your bank to deposit the cash and sign-over the title. Have someone else with you, if at all possible. Most transactions are legitimate, but it only takes one wacko to spoil your day. Be careful, but not paranoid.
Show it clean, with all your stuff out of it, ready for them to pull away. Keep it locked (door, and hitch) while it is listed. Don't post your address, and only give it to someone after you have made it clear how you will do the transaction.
Good luck!
2017.5 Lance 1995
2017 F150 EcoBoost, Max Tow
Most camping off-road