I don't really understand the problem? It would be illegal for you to purchase them in CO? Why not just go to a neighboring state and buy mags? Give them your business, you don't need ID to purchase a mag.
I mean how are they gonna keep track of who's got what?
Not just illegal to purchase, illegal to possess unless you purchased before the ban. I haven't seen the mechanism for proving magazines were purchased pre-ban. Possession of a post-ban magazine will be a class 2 misdemeanor punishable by fines and/or 3-12 mos in prison.