>GOG is the only major storefront where I feel like I actually own the product.

How do we re-sell our GOG games to someone else?

If I own it I should be able to sell it again, right? Like I used to sell old console game disks after I was done with them.

The same way you sell your disks: find a buyer, send them the game files, they send you the money

Just give them the files and pinky promise to delete them yourself?