A computerized, electronic purchase mediating system includes a purchaser database having a list of purchasers and a merchant database having a list of merchants. The purchaser database stores information about each purchaser including a set of personal payment methods that the purchaser could use to purchase goods and/or services. Similarly, the merchant database stores information about each merchant including a set of accepted payment methods that the merchant would accept for sale of the goods and/or services. The purchase system also includes a processor coupled to the purchaser and merchant databases. The processor receives a purchase request and accesses the merchant database according to a merchant identified in the purchase request to retrieve the set of accepted payment methods which corresponds to that merchant. The processor also accesses the purchaser database to retrieve the set of personal payment methods which corresponds to the identified purchaser. The processor then computes an intersection of these two sets to derive a common set of any available payment method that is both accepted by the merchant and can be used by the purchaser for purchase of the goods and/or services. The purchaser is presented with the purchase amount and the common set of available payment methods to choose a most preferred form of payment. Upon selection, the processor consummates the sale and signs a digital signature with the purchaser's permission via password verification to ensure for the merchant that a completed transaction has occurred.