I don't actually have any individual games that could be considered rare or valuable, but I could probably get a decent amount of money from my collection just by sheer volume. I have a good 80 NES carts, all of which still work and most of which are in mint condition even, plus handful of peripherals. But I would only consider selling them if I actually needed the money, and even then it'd be one of my last resorts - I'm enough of a collector that I wouldn't part with them without serious consideration.
Though it's only that collectors value and the geek cred of owning them that stop me. As for the games themselves, selling the carts wouldn't stop me from being able to play them - the internet has made sure that old games will always be available to find and play again, sometimes even legally!