Is it permissible for someone who offered a commodity for sale online at a certain price to refuse to sell it to whoever agrees to that price, out of greed for a higher price, and then offer it at a new price?

If a seller displays his goods online with a specific price and specifications, this is considered an offer directed to everyone. If anyone accepts this offer, the sale is concluded, and the seller is obligated to deliver the goods and is not allowed to refuse. This applies to offers directed to the public through various media; once a compliant offer is accepted, the contract is formed. Therefore, the seller must deliver the goods to whoever accepted the purchase at the requested price, or request rescission from him if he agrees.