sticker under the screw

I'm going to differ with neuroanatomist a bit: just because it was sealed doesn't mean it was new. It's not that difficult to re-seal a box and make it look like it's never been opened. Was this "electronics dealer" an authorized Apple dealer? If it was, then you're very probably OK. If it wasn't, then it's entirely possible that the Mac is not new and was at some point repaired by someone and repackaged.

I would first suggest you go here (if you're in the US, otherwise go to the appropriate site for your country):

Check Your Service and Support Coverage - Apple Support

and put in the serial number of the Mac and see what coverage Apple's records say it has. If it's one year from the date you purchased the Mac, then you should be fine. If it's a significantly different date, or is says that coverage has expired, then you most likely bought a used Mac that you were sold as new, in which case I'd take it back, get my money back, and buy only from an authorized dealer.

Regards.

New Apple products are sealed in the box with a pull-tab (has a little green arrow symbol on it). If yours wasn't, it may have been opened. Either way, there should be nothing visible under any of the screws. If it was sold as new, contact the store where you purchased it.

If it was factory sealed, it's most likely something left over from the manufacturing process. I've never seen that on a Mac, but I have seen it on other electronic items.

As long as it was in a sealed package, you're fine.