I would like to build a 2-sided mobile android app, one side is clients and the other side is merchants. Client has to put in basic information such as his full name, address and ID card number. Merchant has to put in the name of his shops and the products each shop sells with the price of each product. The client can then select the shop he wants, see the products, choose the quantity that he wants from each product, and press BUY and proceed the payment with paypal. Once the purchase is made, the client will receive an invoice on his mobile phone and a queue number. The merchant will receive the same. So then client can walk to the merchant and collect his items by showing the queue number and name of client.