An escrow is someone (company or person) who is trusted by the buyer and seller. The escrow will act as a middleman who makes sure that neither the buyer nor the seller scams anyone. Money, and usually the product, goes to the escrow. The escrow verifies that the product is as advertised, and then forwards both the payment and the product to the respective parties. If there is a dispute, the escrow is supposed to act as a mediator and resolve the dispute.
ok. Now makes sense
But who could be the escrow if I don't have any friend in common with the guy I want to trade. (this is an example)
A lot of people on the forum act as escrows. They have trade histories and are trusted by other members of the forum. The feedback system lets users see who trusts that user and has had a good experience trading with him.