1. Ruthlessness
2. Ability to make people think they're getting a deal and you're doing them a big favor when you're really making a killing off them
3. Offering something that is very difficult to obtain from other dealers (being the only guy around who's got [whatever], that way people will HAVE to come to you).
P.S.----Also, being the ONE, or one OF the only person the main source deals with helps tremendously. If you're the guy that has access to the biggest amounts at the lowest price, you stand to make the most profit. Often times the importer/cook/person with the script, etc. will only deal with one or possibly two people (often for the purpose of expanding availability in other areas), as a way of keeping it quiet that they're the main source, and don't have to deal with shady customers, addicts, possible set-ups, etc. Also, if its semi-clear to others that you're well connected- people will often hesitate to fuck with you or rip you off knowing that you have to power to permanently cut them off or worse. It also helps to have just a few trusted, steady customers than to put the word out that if anyone wants [whatever], you're the person to see. That doesn't rule out random instances (ex. meeting a person at a party), selling to a stranger if you think the risk is worth it, often times these people will pay much more than normal price and will keep buying so that they'll eventually be geting prices, should you choose to deal with them again.
This is all hypothetical, of course