Now now... you say, the dealer has cashed your 
money order? Plus taken your PayPal money..? 
Well, whether he did it on purpose or not, you 
have credit. No matter how long it's been.
Just place your order and say "For payment I'd 
like to use the money order I've sent [date]. 
Circumstances made the money order arrived late 
and I had meanwhile paid through PayPal. So would 
it be possible to pay for this order with the 
money you received from my money order please?" 
or something like that. I had a similar incident 
once and it worked. Absolutely attach all data, 
address and stuff, so he knows who you are and 
what you're talking about.

^^
Anna