What is the ruling on selling over the internet if the seller and buyer agree on the price, but the buyer does not specify a period for receipt and payment, and then does not return to complete the sale? Is it permissible for the seller to sell the item to another buyer?

The agreement with the customer in this case is merely a promise to buy and sell, and it does not obligate either party to the contract. As for specifying the delivery period, the general principle in an absolute sale is that it should be prompt, and delivery must occur upon the conclusion of the sale, unless there is a custom to the contrary, in which case the custom is followed.