I hate the "best offer" option! As a seller, anyways. I once had a card listed for 90 dollars and a guy offered 2 dollars. Maybe he was hoping I would quickly accept without thinking or hit the wrong button?
I would think that if a seller wants 50 with a BIN option, he should list it for 70. Then when the cheapskates roll in, you can counter closer to what you actually want, and you are both winners!