The process of accepted counter-offers moving to pending status on the listing's offers management page is something eBay has been rolling out.
While offers are in pending status the item is available for anybody else to purchase until the buyer pays. eBay is trying to eliminate the inconvenience to sellers when buyers lose interest after an item ends so the seller does not have to start from scratch and lose interested buyers when they relist. They're also trying to eliminate the inconvenience of having your item off the market during the 4+ day waiting period for payment.
If the buyer does not pay within a certain amount of time, the offer will expire and be removed from the pending section. If the buyer pays then it will become an official transaction on your orders page. In the meantime, any buyer can purchase at full price or make an offer.
