What would prevent it? In Illinois gun owners are licensed so if you weren't, it would be a crime to hold the gun. What prevents it in CO?
CO requires all private transfers to go through an FFL for a background check.
Problems with the law? Oh, where do I start. Say I go on a week long business trip. Does that mean my wife is now "in possession" of my firearms? Do I need a background check on her? What about my standard capacity magazines for my AR? We're no-longer supposed to buy/transfer mags with capacities over 15 rounds. Is leaving them with her an illegal transfer? (she loves my AR BTW, keeps it by her side of the bed...sigh) What if my wife and I go away for a week to celebrate our anniversary and have a friend stay at the house watching our pets? Do we need to simply put away the firearms we don't take with us? Lock them in a cabinet? Trigger lock them? What if we happen to trust our friend with our firearms and want him to be able to defend himself? (guess he could bring his own with but...)
The recent laws regarding magazine limits and transfers are idiotic and I know, I know {wink wink} they are being ignored and circumvented. Heck, our County Sheriff stood up in public (multiple times) and said the magazine laws are unenforceable and he has no intention of wasting any time/resource even attempting to enforce it.
(*) That is, provided that to the best of your knowledge the person is not legally barred from possessing a firearm.
It is illegal to transfer a gun at all without a background check now. Hence I would have to convince said friend to visit a gun store with me and then legally transfer it.