I'm surprised GD even allowed a domain in GD Auctions with an extension they don't support to begin with. The opposite happens, that an extension they support like .co.uk or .mx is still not allowed in GD Auctions, just to name a couple of ccTLDS. I've never had your case but the buyer has to accept a push at the registrar you have the domain. So no, the buyer cannot claim he did not get the name. If he does that explain to GD the case and that it's not your fault. And if GD denies you payment in the end call them retards and expose them on DNF for having let you list an unsupported extension in their auctions
:doh: