ask him to come explain here. As he did checked the pin during the trade, let him explain how the pin got broken.

I have had similar problem myself with a motherboard. I sold took the board back from warranty (my fault didn't test it) and then sold it to someone in forum with poslaju. He then told me the board is not working. I ask him to sent back to me and I refund him the money. I made losses from bearing poslaju cost sending to him. Luckily he beared the cost for sending back to me. Anyway, I then tested the board and it's working 100%.

Well, what can I say....sometimes unlucky to get noobie buyer. BUT, in my case, the board still have warranty. In your case, broken pin means warranty void. So...goodluck.