I think it may be poorly worded, but all it is really telling you is that, if your item is a fixed price item and you have it set for "immediate payment", that only works when the buyer pays the full price. If the buyer makes an offer and you accept it, immediate payment is no longer active and the buyer has more time to pay. Some pay very quickly, some don't. After two days of no payment (I think it's two, not 100% sure), you can file an unpaid item claim with ebay. Or you can just send the buyer a polite reminder first, to see if that will result in payment. Either way, if they don't pay after a set amount of time, the transaction can be cancelled , your fees refunded, and the buyer gets hit with an "unpaid item strike" which goes on their permanent record.
