The  seller does not know an item is faulty until they are advised of it, and of course it won't be in the description.

Bear in mind that shipping replacement arms from the US (including postage) is costing more than the entire original purchase - again including postage

 Bear in mind if you had done the right thing first and contacted the seller this expense would not have been incurred by you so you have no grounds on which to request reimbursement for this.

Had you gone through dispute resolution paypal would have asked you to return to seller at your expense by trackable means, and the seller has stated he would have refunded no questions asked.

He has done everything required of him by eBay and paypal but you have refused his offers and will punish this seller by leavng a red dot.

Not good form, sorry, but you did not deal with this in the correct manner at all and the seller should not be punished for your lack of knowledge on eBay.