There is nothing wrong with outsourcing a business task. External linking is no exception.
Like anything on the various outsource networks, you need to make sure you have a way to monitor the production.
The last thing you need is someone using blackhat techniques to do a white hat mission. Your site will end up in the search engine black hole.
There are a number of white hat services automation services that do this, and the task of using the service is time consuming and can be outscourced through the various outsourcing networks.
Just use good due diligence.