You can sell hand guns and long guns to any resident of your state who is of legal age and is not further prohibited from possessing firearms. There is no requirement to do a background check or keep a person's name for any length of time. At its simplest, the transaction can be cash-and-carry with a wave goodbye. At a minimum, I ask for proof of age and residence (driver's license). Some buyers want a Bill of Sale, or some sellers require one as proof of the transaction. You can sell your firearm for a profit, but if you engage in a lot of transactions you may be deemed to be "in the business", for which you will need an FFL. Onesy-twosey sales for profit are OK. It is OK to rotate your personal collection and nothing requires you to sell at cost or take a loss.
As for your liability, you have none after the sale -UNLESS- you live in a state that uses nebulous language that requires you to "reasonably know" that your purchaser was inelligible. But who can know that? I know of no such language in Colorado.