What magic wand do banks possess that enables them to declare-into-existence a merchant account, anyway? I mean in a technical sense, not a legal sense. What are payment gateways and other banks seeing, that allows them to know that a particular merchant account X is a "real" account, that can be targeted with credit card payments, rather than a made-up one?

Is there an X.509-based hierarchical bank registrar system for charge-origination signing certificates, for putting charges onto the card networks? Is there a DNS registry of merchant accounts for pre-checking charges before attempting them? Do banks underwrite other banks into existence by signing their certs?

Pretty much none of that. What exists is a whole lot of money, a willingness to lose that money if a bank makes bad bets on the trust and security of a customer, a bunch of laws to adhere to, and willingness to go to jail if those laws aren't followed (or, perhaps willingness being the wrong word, an understanding one will go to jail if those laws aren't followed and protected parties lose money as a result ;) ).

In short, it's a different category of risk than the category that an individual business or even a business that is acting as a middleman for transactions takes on. And it's a different category precisely because most of the solutions aren't technical; they're legal, social, and financial. If a bank gets screwed it just gets screwed; even if the law intervenes to deal with transgression, money is often just gone as a result of such fraud. So they are comparatively more conservative in their decision-making.

Banks have a whole lot of bank laws. While mostly it is paperwork, it also means some trust and responsibility that can be used to do things that banks do fornon banks like process money. Check with the local laws. The law protects customers of banks at the expense of the bank, which is what you want customers to know before using you for bank like things.