I recently bought a bunch of N64 games at a flea market and on one of the games it said in a red box "NOT FOR RESALE". Specifically, I'm wondering if it was OK (legal) for me to buy this and more importantly, legal for me to now sell it?
The short answer is yes, it's perfectly legal for you to sell the game. Any game with the words "Not For Resale" on the label usually means that it is a game that was a Promotional, a store Demo, or that came packaged with something else and was not to be sold separately. It could have been part of a maintenance cart, a special edition, box set, etc. From the retailer's standpoint, it originally meant the retailers weren't allowed to sell the particular SKU individually or separate from the set or special edition that it came with. It was part of a bundled SKU, an employee NFR version or something like that.
A side note, these games today are generally much rarer and thus much more valuable (depending upon how many of them were produced) than the common game titles. But yes, selling it now is perfectly legal and acceptable since it is considered a collector's item, not a bungled new retail item.