I think it’s a matter of contract law, in real life. By not allowing scams, CCP is declaring the bazaar as “out of game”, and working in a system of real-life deposits and partial payments will open up a whole lot of potential litigation.
Its specifically in the rules that you cannot do payments
Down-payment plans are not supported by CCP Customer Support. As a seller, you are agreeing to a certain amount of ISK only, which is to be paid in one payment. Once again, any forms of scams regarding character sales are strictly prohibited.
My opinion, if you were allowed payments, the char should be moved to a ccp controlled acct until all funds are paid.