Basically Freelancer acts as a 'middle-man' between the freelancer and the buyer. Buyers must make deposits directly to Freelancer, it can be via PayPal, Credit Bank, Bank Wire and many other methods... once they receive the money, they will credit the buyer a 'virtual currency', the buyer can place it on hold until the project is completed (escrow type), and release the virtual money to the freelancer once they completed the requested job.
So basically it's not just using Authorize.net, PayPal, MoneyBookers as your payment gateaway. It must be complimented by
server-side scripting (PHP, Perl, ASP) to complete the full escrow function. You need a code to manipulate some sort of 'virtual currency' like Freelancer.com does.
I would recommend you to hire a web developer to get this done for you, or look for a pre-made solution (which can be cheaper).
Hope this helps you