There don't seem to be too many Mac users in Poland - not like in the States. I have heard that Macs are about twice the price of PC's over here, so that'll eliminate a lot of customers right there...
I too am unable to navigate Allegro because of the language. I have some RAM to sell, but I don't know even how to start. I guess finding a computer-savvy Polish-speaking buddy might be a start.
Also - check out Gumtree (warszawa.gumtree.pl) - you'll face the same language issues as you do with Allegro, but it's another avenue. Good luck!
I don't think the keyboards are different here than in the states.
Anyway, because of the added VAT you paid (22%?), you likely won't get anything near what you paid for it if you sell it in America.