A buyer who clicks the Buy It Now button in your listing is directed to pay immediately for the item using PayPal.
Until the buyer completes payment, the item remains available to other buyers to purchase for the duration of the listing. The first buyer to complete their PayPal payment officially wins your item.
Once a buyer completes payment, the listing ends (or is decremented by the quantity purchased in case of multi-qty fixed price items).
Find the answer to your question
I list fixed price items requiring immediate pay on eBay. I want to make use of the BestOffer feature and therefore listed an item with the same. My AddItem response contains the following warning message
<ShortMessage>If this item sells by a Best Offer, you will not be able to require immediate payment</ShortMessage>
What does this mean?
Listing an item requiring Immediate Payment means that payment must be made immediately through PayPal after committing to purchase the item.
How it works
If an item requiring Immediate Payment is also listed with the Best Offer feature, the immediate payment requirement is not enforced when the sale happens via Best Offer. Buyer makes an offer and the seller has the option of accepting, declining or making a counter offer. When the seller accepts the buyer's offer, a transaction is created. Thus the immediate payment requirement is bypassed.
Also, if you have set BestOfferAutoAcceptPrice when listing the item, a transaction will be created as soon as the buyer makes an offer that is greater than the BestOfferAutoAcceptPrice.Thus the immediate payment requirement is bypassed. Also, you will receive the following warning message while listing and item with <AutoPay> and <BestOfferEnabled> set to true.
ShortMessage: If this item sells by a Best Offer, you will not be able to require immediate payment.
Thus listing an item with Immediate Pay and Best Offer could result in transactions which haven't yet been paid for.
For Immediate Payment listing, unless the payment is completed and eBay is notified by PayPal, the Transaction is not created. This means that the listing is still available for another buyer.
Whereas, for BestOfferAutoAcceptPrice listing, offers are automatically accepted when a buyer submits a best offer that is above the BestOfferAutoAcceptPrice. Once the BestOfferAutoAcceptPrice is accepted a normal transaction is created (which is similar to normal commit to buy). At this point the Immediate Payment feature is disabled and the transaction is treated in normal checkout flow.