I think prepayment presents a problem also. What would be the deadline to prepay, before the start of the swap? Then you would have to stop taking new sign ups at least a week before the swap starts, to give everyone time to get it in. Or else people would be joining a day or two before the swap and it would be difficult to get payment in time. If the deadline were after the swap started then it would defeat the purpose because they would have already start requesting and giving away seeds. At any rate that is a choice that should be left up to the host.
