All you need in CO is to show proof of age/residency (iirc) Make sure they're a resident of CO, and of legal age to own what you're selling.

That being said.. Most of the people I have had dealings with.. a valid Concealed permit was good.. even a DL..

It's all how you want to do it, and if the buyer is willing to go your route.

Few months back.. this person who will remain nameless... got a ration of shit for asking for copies of DL and what not.. while no one I know of did.. someone agreed to his terms.. It wasn't even a great deal.. but I digress.

It's all what you feel comfortable with.

When I sold my gf's xd on here, I explained the laws to her, and let her decide what would suffice. Also made sure she was present at the deal. This scary asian guy showed up in a mini-van.. he was totally sketchy!

Even with a check run..there is no guarantee that the person purchasing it is mentally fit (Jared Loughner) to own a firearm.. I mean it wouldn't be PC to classify people as mentally deficient now would it.