Many years ago, my father befriended a pawn shop owner. My father talks with many people easily. My father asked the Pawn Shop guy how he could know what so much stuff is valued at. The Pawn Shop guy responded "the deal is in the buy." You can always pay too much for something, and give away your profit, but if the price agreed upon is sufficiently low, then there shouldn't be much difficulty selling the piece to others.
This can be applied to real estate, or other money lending schemes.