In this case, buying in general (not only in prostitution) should be banned altogether as the seller might be forced and you're saying that it can't be proven t

In this case, buying in general (not only in prostitution) should be banned altogether as the seller might be forced and you're saying that it can't be proven that the seller is not being forced to sell. That doesn't make sense and you know it. Of course it can be proven up to a reasonable doubt that the seller is not being forced. That is how society and the civilized world works, not by removing 100% doubt but by trust between its citizens.