Fair play to anyone who is fortunate enough to own a rarity, but even is this were legit, even the most avid collector would be hard-pressed to part with half a mill to own it, especially with faking so easy:
1) Take 1 old atari cart
2) print a label off on sticky paper
3) stick it on
4)For added accuracy, hack the insides with a flashed chip using a downloaded rom
5) Tag on an outlandish story of how you acquired it and put on a ridiculous price tag.