What a lot of people do not understand is that the O.P is asking to have the option to use immediate payment, not to have it where everyone was forced to use it.
There could be a box to check in the listing form if a seller wanted to require immediate payment & those who do not want to use it would just leave the box unchecked.
Given all the years they have supposedly been going to have an API & mobile app which still has never happened. I would have to say we will probably never see an immediate pay option.