For an auction it is easy not to be near a computer at the end time etc, forget you have bid, not expect to win, so a prompt may be justified.
When someone uses a BIN there is no logic to it not being paid there and then.
If you cant afford it, don't buy it.
If you want to do other window shopping, dont buy it until you are sure, ready.
It is very annoying that when people use BIN and don't pay as they are on the computer there and then to provide payment...
(I know it can occasionally happen that internet is lost after a purchase, however I do not think on the scale of the times it happens.)
(Or a polite note to a seller if there are problems with payment.. is not unreasonable, few items include the time cost begging for payment

)