You have to be involved with the trading community for years, become trusted, and slowly work your way up from small to large transaction sums. The only reputable middlemen in the Combine have completed trades worth billions of credits without so much as batting an eye. Some factions have "insured middlemen" lists, where traders would be reimbursed if that middleman stole in their capacity as a middle. The only other way is to be sponsored by an already-listed middle, such as Syn, Dac Kain or Togan, but they would really have to know and trust you in order for them to do that.
In essence, the only way to become a middle is to start middling, but you're going to have a hard time finding people willing to accept you for anything other than for paltry sums until you've been trading for several years.