That's too bad, I'm so sorry. I have a similar situation, the closest LFS
is terrible... I have to go an hour away to a great one, but it's definitely worth it!
Is there any way you could re-home a male, and then go back and get a female? Maybe a friend/family member has a tank and would be willing to take him? If this is not an option, I know you said the LFS
likely wouldn't take him back, but maybe if you try giving them a call and explaining what happened, they'll change their minds? It's their fault, after all.
It's sad that the pet store employee put you in this mess by not knowing how to sex platies properly. It would be terrible if they got stressed or injured because of that employee. You asked for females and they messed up, so I don't think it's fair that they blame you for their negligence.