1. A computer system comprising a mobile computer program application, an at least a first computer device, and an at least a first server computer device, wherein: said mobile computer program application is operably installed on a buyer mobile computer device, said mobile computer program application comprising executable program instructions instructing said buyer mobile computer device to receive a transaction-specific identifier and transmit said transaction-specific identifier to a seller computer device; prior to a sale of an at least one item by a particular seller to a particular buyer, said at least a first computer device programmed to receive a buyer request by said particular buyer transmitted by said buyer mobile computer device for said particular buyer to be able to purchase at least one item featured for sale by said particular seller recognized by the computer system, and in response to said buyer request, generate a transaction-specific identifier, and make said transaction-specific identifier available to said buyer mobile computer device for a particular purchase transaction regarding at least one item to be sold by the particular seller; said at least a first computer device further programmed to store in a computer-accessible memory a record comprising a relationship between said transaction-specific identifier, a buyer-specific identifier, and a seller-specific identifier; said executable program instructions instructing said buyer mobile computer device to provide said transaction-specific identifier to said seller computer device; said at least a first server computer device programmed to receive from said seller computer device an identifier of the transaction, an identifier of the buyer, and an identifier of the seller; said at least a first server computer device further programmed to compare the identifier of the transaction, the identifier of the buyer and the identifier of the seller with said transaction-specific identifier, said buyer-specific identifier and said seller-specific identifier; and if said identifier of the transaction matches the transaction-specific identifier, and said identifier of the buyer matches the buyer-specific identifier, and said identifier of the seller matches the seller-specific identifier, then said at least a first server computer device further programmed to confirm that the buyer has authorized the particular purchase transaction.