First course of action is to block that account from buying anything from you so they don't try to get back at you by purchasing and returning your stuff. Then reply back with what phil said.
While I wasn't brave enough to send back the close-up of the 18+ and a "NO" Trump meme, arguably the best response for a situation like this, I did give the guy a sensible reply. Against my better judgement I might add, as usually these people can't be reasoned with. I blocked him just in case, of course, per your suggestions. His reply to my message tells me how little the average buyer knows about what it costs to resell.... I understand his frustration and it does have me thinking twice and scooping up new releases. It is certainly the "ugliest" side of reselling. I gave him an even calmer reply and explained that $325 would roughly be the typical reseller's break-even sale price.