An escrow is a third party who would hold all of the items to ensure honesty. For example, let's say that you're selling a building block for BTC1. You would send the building block to the escrow, and the buyer would send his payment to escrow. Escrow then ensures fair play, and sends the payment to you and ships the block to the buyer.

ALWAYS use an escrow if theres any money involved or anything sounds shady