Do they buy 5th wheel hitches?

I would speculate that it would only be if it was part of a trailer purchase, along with ur trade-in. But my experience has been that no place will give you the fair amount. You're better off selling it privately.

Np unless it's part of a trade

I agree sell it privately you will get more money out of it

Yes

I believe they wpuld only sell new hitches so i doubt they would buy used ones

If they do, it would probably be under the low trade mark. I would Try and sell somewhere else first.

Put it on a Buy-Sell-Trade site or look for a trailer site that will let you list it.

Ditto