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So, unless you have a network of sites and the knowledge on interlinking them and understand the risks of doing it less than well - you won't be able to duplicate what those SEO companies are offering on your own.
And I'm not even saying you would want to. Again, a lot of different factors and risks at play.
"build up the traffic"
Also, building up traffic and building up links are two different things. Some SEO's have generic sites that can funnel traffic to any client they choose.
Here's a thread about some of the perils associated with outsourcing [webmasterworld.com].