Reef Central is mostly people who don't know how to keep sharks. I've seen too many "I just bought a shark for my ____ (insert tank size that is way too small), what do I do next?" threads over there.
Try the SW classifieds here on MFK.
Shark Ray Central would be good as well.
Just make sure that the buyers know what type of horn they are, and that most horns are cool/cold water sharks and will need to be chilled down to do well long term.