Quoted from vidguy:

Would be pretty easy. If the Buyer later decides to sell the pin, he first contacts the Seller and says, “hey, I was thinking of selling for $x. Are you interested?”
The Seller has the option to buy back at that price. If they can’t reach a deal, Buyer is free to sell to others.
I’d be interested to hear others experiences. It seems like half of Pinside operates this way.

Quoted from frenchmarky:

What good it is if the buyer offers to sell it back to you, but for a million bucks?

Quoted from CrazyLevi:

Never. Just a horrible idea. Could turn friends into enemies and enemies into bitter rivals. Just think about how you would enforce such a ridiculous clause.
Don’t sell a game unless you are willing to not care at all what happens to you after you sell it. When you sell a game to someone, you aren’t renting it to them or asking them to babysit it for you, you are selling it.