That's a tough one.
I hate to oversimplify but I really think that trailoring is a "must know" for most horses. She will be worth 2x the price if she will just get in the dang trailer!
I understand your frustration but it really is one of those things that is a deal breaker...at least for me. My advice is to teach the horse to trailer...I don't know how to sell one that doesn't...HOW will the new owners get it home??? LOL...what a mess huh?! I sure do feel for ya!
I know its not the "preferred method" of training...BUT, can you feed the horse in the trailer for a while? I'm out of ideas other than that.
I wish you good luck!