As any lawyer will tell you, "It depends." In general, I would think that it is safe to say that it is illegal.
Depending on your state's penal law, you could be charged with acting-in-concert with the steroid seller. In New York, "When one person engages in conduct which constitutes an offense, another person is criminally liable for such conduct when, acting with the mental culpability required for the commission thereof, he solicits, requests, commands, importunes, or intentionally aids such person to engage in such conduct." So by referring someone to a seller, you are equally culpable under the law.
I don't practice in federal court, but I'd posit that any AUSA worth his/her salt could obtain a conviction for you for conspiracy. Conspiracy is the easiest crime in the world to prove.
Now, I say "it depends," because every case is fact specific. There are factual scenarios where someone may not be found guilty of conspiring or acting-in-concert. For instance, New York has a defense called "agency." Agency basically means that you were just helping a buyer to obtain drugs; you were the agent of the buyer. The flip-side of agency is being a steerer; you are the agent of the seller. The line dividing agency from steering is a very fine line, and many defendants find themselves on the wrong side.
So you should be aware that you could be charged with a crime just for referring someone to a seller.
Genarr3, thanks for referring me to the thread. I can miss threads at times, and I enjoy being able to help people out in the legal arena.