It is against the rules and you can't do it. I am pretty sure it will be obvious too who sold and for how much and besides why would you want to risk a ban for merit points? Write quality posts and everything will be fine. This helps get rid of the spam, this is what we all want isnt' it? 
And how to distinguish a quality message from a non-quality one? After all, this concept is quite subjective. I, for example, may like this your message, I will send you a few units of merit. Another will consider that this message is generally of poor quality and therefore suspected me that I sold these services to you. Is it still someone I have to prove that I did not sell them, or what? And how do you prove that a person sold other services to someone else, if there is no obvious evidence? All this is not easy.