Welcome, Dapper Dan.
That’s a good question. (As is often the case, that means a question without a good answer.) For what it’s worth, the OED records the verb first (to fence = to sell stolen property) in 1610, and the noun ("a receiver of stolen goods") not until 1699, although with thieves’ slang one can’t be too confident in the dates.
No indication of how these senses developed is provided. I could suggest that it relates to the idea of the fence providing separation and protection for the thief and the ultimate buyer of the goods, but that is purely speculation.