This is a prickly, actually..if not morally, then logistically.
If I were in OP's shoes, I would also want the quickest way out, and least complicated....given I had done nothing remotely wrong.
But opening a case with eBay and forcing a return to the seller ?? ...
OK so it gets the horn out of the OP's hands....but ostensibly it puts the horn back into seller's hands which may be returning stolen merchandise to the third party (let's assume the pawn shop had no idea it was stolen).
Given RS has already requested the OP to send horn to them.....would this get the OP off the hook here ?
Or would RS be angry at OP for returning the horn to the eBay seller ?
You see ?
I say, do NOT open a 'request return' case....but DO contact eBay and speak to a rep and explain the situation, emphasizing that RS instructed you to send horn to them. Then see what eBay says....
I doubt you will lose your $....I wouldn't worry too much about that...BUT you should try to find out the best steps to handle this via it being an eBay transaction....I am certain this happens on eBay and they have a policy path for it...
The second option would be for you (OP) to discuss with RS that you'd be happy to ship it to them but before doing so you want your $200 back. I think this is a BAD course, personally, because it starts getting you unintentionally entwined in a stolen property situation, as opposed to eBay being entwined in it...
IOW, I think a lawyer would probably say your transaction relationship is with eBay and the eBay seller, and NO other parties...so do not open/pursue a further relationship with a third party (which in this case would be RS) on the matter....