This sounds to me like another one of the other site ideas being suggested here.
For myself I would NOT like to see it as a compulsory addition from a sellers perspective and do not like it from a buyers perspective. As a seller I never used it on the other site even though it was an optional extra. As a seller, it would put people off buying from me, especially if they wished to buy more than 1 item. As a buyer - if I wish to buy a couple of items from a seller I find the 'pay now' option a deterant and will walk away and shop elsewhere.