The question is how you prove that you own something (physical or digital). With GOG even now it is not really clear. Copying or forging these emails you get is a bit too simple. So if GOG accidentally forgets that you bought 400 games with them for $3000, you face a huge financial loss and will have a difficult next to impossible time to be awarded your rights. If it would be easy, probably everybody would say that GOG owes him some games. But GOG games you can backup. What if Steam accidently forgets a customer? Or if you buy a license for access to a valuable database and they don't let you in for some reason.
What if your bank forgets you had money in an account? After all this is the most essential and convertible form of digital good. It's just a number somewhere and can so easily be adjusted.
That's why you keep receipts, credit card statements and other proofs of purchase. Never delete those emails, and you are fine. If you use things like PayPal you will also have nice lists of transactions. Bank statements show you your transactions. The onus is on you to make sure it is OK, and if you find something wrong, you contact the bank/seller. If they do not fix things, then you take it further. They are regulated, but it is still up to you to make sure things are going correct with your money.
The bank lock your money during a transaction, not any outside agency, they need an court order (or what it is called) to be able to do so. Banks also transfer money between themselves, as do not work in your previous example. Money is not a good, and not really transferable example. It is a concept. it is like trading in the concept of a game, which then can be redeemed at store point (Duke Nuke'em is worth 5 game, Postal 2 is worth 1/100 game, and so on), and the DD's do not transfer the licenses, but "Game" between themselves - but this is not how things work... does it make sense?
I find it difficult to judge how much more abuse there would be and if this would be significant.
mm - the problem is that this will have different impact than piracy, and publisher and sellers will most likely be more scared of it. It is hard to tell, but as you might have guessed - I am pessimistic. Best case scenario as I see it? we will see increase in game prices... worst case? more subscription based gaming and heavier DRM. Basically, I do not see a single positive outcome in the long run for resale of games. Short term, yes, I get some money for some games I do not want any more right now - but is it really worth it for the long term consequences? I think not.
("Or if you buy a license for access to a valuable database and they don't let you in for some reason." - this is surely DRM?)
(P.S.. "Prediction is very difficult, especially if it's about the future" - Niels Bohr.)