I'm inclined to agree with Poly, If it says Mega Drive and not genesis its definitely not a US/Canadian version unless it is a misprint. I have 2 copies of Sonic 2 and both have Not for Resale on them, but they are not stickers, its just printed on the game label to look like a sticker. All the ones which say not for resale should of been the copies packed with the Mark 2 system.
Think about this...I don't often pick up loose games, but sometimes I come across people with empty cases and ya just never know....whoever donated that game to the salvation army could of had the pal version loose and stuck it in a US case...although, from my research..I haven't been able to find any PAL versions boxed with the Mark 2 system or without that has the Not for Resale graphic...