In Colorado, Hi-cap mags are completely illegal to sell PERIOD. Ownership is grandfathered into the law for folks that owned them prior to the law.
You can't buy hi-cap mags in this state, nor are you allowed to possess them in this state.
2 years ago, I went to the Tanner gun show and traded a stainless, tactical Mini-14 on another rifle. I had 4 30 rnd mags with it. The vendor said he'd love to have them but legally couldn't touch them. I actually had to be escorted out of the show with the mags and secure them in my car before I could come back in...... apparently they were supposed to have caught that at the door when I entered and had the gun checked. LOL. Now I'm stuck with 4 mags I can't sell because for anyone else, they're illegal.
You can purchase and posses them in the non hi-cap restricting states, but bringing them into Colorado is your risk.
Mind you....... Hi-cap mags manufactured after the law are required to be date stamped. Pre law hi-cap mags will not have dates. So, there's really no way to prove that you didn't have them prior to the law....... if you dig what I'm saying.
So, if you buy and possess date stamped hi-cap mags in Colorado...... you got no excuse and you're chargeable under the law.
Also buyer beware......... ATF does buy/sell stings using the guise of private party face to face transfers of hi-cap mags. KNOW who you're dealing with if you decide to take that risk.