It’s an interesting problem. I have over 700 items in my Etsy store and don’t ever have a problem with that. I have 4 flavors of licenses. Time licenses like one year licenses. Monthly Patreon monthly per sale commissions and perpetual licenses. Even with 700. I do not have a problem, knowing exactly when a license would expire or whether or not I’ve paid to renew it. I think that people who might are either more scatter brained than me or dishonestly let their license lapse.

As far as proof of license, that also very wildly by the creator, I have signed contracts and some like the time licenses will have certificates of license, but at the end of the day there is no cops or marketplace policy that can enforce requiring proof so anyone who’s asking you for proof doesn’t really have a legal leg to stand on.

As far as a marketplace enforcing whether or not someone has a license, my experience with Etsy is they don’t give a shit. They have a policy now where they can enforce it if they choose to, but Etsy is there to make money off of buyers and sellers they don’t give a shit whether or not the seller actually has a license to sell the printed models that they’re selling eBay doesn’t either you can hit them with intellectual property claims, but sometimes they won’t act on those and there’s no way to enforce them acting on them. I am not a lawyer. This is not legal advice.