Before on eBay, a buyer could buy an item and the item ended immediately after. Now eBay has changed this system to where when a buyer sends an offer and the seller accepts it, the buyer has to agree to commit to buy. The buy it now items are still live when a seller accepts an offer. This means if you are selling, if a delinquent buyer doesn't pay, another buyer could send a offer and buy the item at the same time.
If you are buying, you can submit an offer, the seller accepts it, but you don't have to commit to buy it.