I know their policy is no returns on helmets, but I'm wondering why they sold you a helmet that's too small in the first place. That's one of the reasons to go to a dealer instead of buying on the internet - personal service. You should go back there and speak to the parts manager. Are any of the pads replaceable so you can get a better fit? You might call Arai too to ask their advice.
I don't care for Motions too much because they tried to sell me a part for my Arai that was under warranty. They even told me they called Arai and were told it wasn't covered. I called Arai myself and got the part for free.
If you really are stuck selling the helmet, try Ebay and GSB too if you haven't already.
"We didn't trade shoves; I did all the shoving." -Bobby Knight, 1981