How about this: "The next user who posts after me, agrees to pay me 0.1 Bitcoin." That sounds like a casual agreement to me. But it shouldn't be allowed!
Then no one will be agreed with your demand. But if anyone feel comfortable than still the person may choice it. I think no one will choose it as there is no benefit/profit. As no one will buy anything @1000$ which real value is 5$. On the contrary if you say I will give 5$ to anyone who will be the next poster of me. By posting after you will not make me loser so I And everyone will accept your offer as everyone can want to buy any product at 5$ which real value is 1000$ or more than the asking amount.
Actually anyone can accept your offer of the person has benefit/profit.