I would take the trade. Single gene females will be as out of style as single gene males pretty soon. I don't plan to add any more single gene females and will be phasing them out over the next year or two.
Sucks that it turns out to be a male. I'd be upset with the seller, but mistakes happen. Now if you got a male and it turned out female, that would be complete negligence. The other way around is somewhat understandable, especially if they bought her as an adult female. You assume someone has double checked by adulthood...