If you sell someone stolen property but you are not the original thief what crime is that?

The "original thief." Well, there is a concept in common law that prevents a harmless middle man engaged in a business from being liable if he/she acquires property in good faith and without knowledge of it's theft. Since you obsviously know the item is stolen, you probably shouldn't be asking questions such as this online. However, in most states, if you take property that is stolen and help the thief in selling the item, you are an accessory. You can be held accountable for this.