Craigslist. I was reluctant at first and not sure if mine would even sell that quickly but it did. Very quick. Went to the first person to look at it. He showed up with cash and I had a clean title. No effort.
This last point above is important. Without a clean title, many would be buyers will pass. If you can swing it, I'd recommend paying it off just to clear the lien. You can always pay yourself back from he sale of the trailer.
Btw, purchased our present trailer on CL as well. We lucked out because the seller had a problem with the title and another buyer. They walked, we weren't in a hurry and when it reappeared on CL again, we jumped on it.