The E on the esrb logo looks correct, you can see the top-left gold square through the case. The holographic sticker effect appears to be correct, and this one has numbers imprinted in the label (beside the N of pokemon and above the official nintendo seal) Looks as legit as it gets. Now you just have to find one that actually looks like that. I made a post here: http://p.tl/qgpm (gbatemp) that describes how you can tell with most GBA games.
Additional info for R/S/E is that Emerald should always have an internal battery that's outlined with light-blue plastic. Ruby and Sapphire have the red plastic. If you find one with yellow plastic, it's probably fake.
I don't have any experience buying these off amazon, so I'd just try to find the sellers that will communicate with you. Ask them for pictures of the item you'd be buying before you buy it.