IMO, they won't come door to door. They'll just make them illegal to own/use/transfer/etc. after X amount of time when you're required to register it. If you don't register it and you're caught with one, then your shit gets turned upside down and you're prosecuted. Time will tell.

As for now, you can legally sell to a fellow Colorado resident who is of legal age (18 for long guns, 21 for handguns) and is not a felon. I generally ask to see a CO ID, preferably a CCW, but do not require a bill of sale. There is no legal requirement for you to keep a paper trail of a private party sale at this time.... that is subject to change in the VERY near future I'm afraid.