This is based on Ropsten test network, get free test Ether here -> Faucet (https://faucet.ropsten.be/)
Ethereum Supported Web Browser, Meta Mask Web Extension
Seller creates a contract by signing up for a transaction, sends the ethers their wish to sell the product for (the ethers are held by the contract), Contract will release these Ethers when either the product is sold or when the seller end the contract, remember the contract should be in 'Created' State to End the contract. After the transaction is completed copy the contract address and give it to the buyer, Ask them to click on 'Yes, Confirm' Button to change the state from Created -> Locked, The moment buyer confirms of receiving the product. The Ethers are released from the contract to Seller's address.
Note - Don't lose the contract address, You might lose your Ethers permanently.