All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
According to your question, it seems that the problematic issue is the delay in delivering the item to the buyer and whether or not it affects the validity of the sale transaction and is an impediment to its conclusion (i.e. it renders it non-binding). If this is the case, then the answer is that it does not affect the validity of the sale contract, even if it was not a Salam transaction (a payment in advance sale). The seller still guarantees the absent commodity (that is not present in the session of the contract) until he delivers it to the buyer. According to Al-Mughni reads: “A seller guarantees an absent commodity that is sold based on description or was inspected prior to the purchase session until the buyer receives it.” [End of quote]
The condition that some websites stipulate for the commodities’ owners, that if they do not send the commodities within days, they will return the price to the buyer; this does not affect the validity of the sale transaction because these websites act as mediators between the person who requests to buy the item and the owner of the item, and they seek to preserve their credibility with the customers.
Allah Knows best.