this business about buyers completing there bargain as soon as they have paid is wrong they have only just entered a contract with the seller, nor is the sellers bargain completed when they post the said item, the contract in both cases has just started, as you all mention problems can and do arise,

sellers need to evolve just likes the laws of on line selling , depending on what you sell the time periods for returns, customer receiving goods, the time period stated in sellers terms, and legislation regarding reasons for returns , if you have a unreasonable buyer for whatever reason and you have left FB for this buyer contradicting your experience with the buyer, your fb left is misleading ,