After the payment has been made by the buyer, the seller can use https://bitref.com/ to check the bitcoin address to confirm if the funds are available and the amount is right. If good then the goods or service will be delivered to the buyer after which the buyer will confirm and then give full access of the bitcoins wallet to the seller to get the payment. But if the good or service rendered is not o the promised quality then the seller buyer will have to return the good or reject the service and then the seller will grant the buyer access to their money.
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