An offer is binding if the seller accepts it. 

If is conditional on being accepted within a time frame determined by the one making an offer, and the seller accepts the offer after the timeframe providing then it is not an enforceable sales contract.  

It has some parallels to sellers putting terms and conditions in their listings that are not always in sync with ebay practices, for example, a seller putting a 2 day payment limit in their listings, nonetheless, it becomes a condition of a sale contract, even though ebay system allows for 8 days in total for the non-payment process.  

IMO ebay outline a range of parameters for trading, some are quite clear cut rules, and others provide guidelines to practice that have a level of flexibility.