It's a little scary how many people have answers for this (myself included).
I would become a one man "Dark Brotherhood" from The Elder Scrolls Series. Except instead of having to preform a ritual of darkness to summon me, I'd just set up a website. Drop me an email, telling me who they are, why I should kill them, and how I should do it. Then I name my price, you pay me, and that person gets iced. I'd have a few rules, just to "keep it Clean".
1. How they die can not endanger other people. For example, no ordering someone to drive a bus into a mall so that the bus explodes, even after hours (think of the Janitors, man.)
2. If you send me false information (example of what is sent: THIS GUYS IS A RAPEEST AND NEEDZ TO GET ICED!!) and my research [which I would do before every contract] says otherwise (the guy groped a woman once, said it was an accident, and she still sued him but lost) then I won't accept. You would no this by my never sending you a price.
3. Political or Religious figures are off limits. This applies to everyone but other governments (for example, the USA contacts me to kill Bin Laden, or North Korea wants me to kill Obama, or something) Even then, I still won't accept based on that alone (like North Korea).
(The only exception to the rule above is if anyone tries to arrest/kill me. If they try, then whoever sent them [China, USA, France] is going to be surprised to find there leader dead for it)
With those rules, "Darkest's Death for Dollars" Bounty Hunting site would be open for business.