Do the freelancers already come with their own portfolios and reviews from other sites? If so, you might want to leverage this initially.

At the same time, most of their concerns will likely be around payments, security, etc. Make sure you are following best practices regarding secure payments, etc.

From a messaging perspective, you'll also want to tie the freelancers skills to benefits that they'll have when ordering from you.

Some things to consider:
1) How much will they save in terms of time? Money? Etc
2) What can your platform offer that no one else can?
3) What are the benefits of outsourcing?

If you'd like to brainstorm some more, feel free to check out my profile and let's jump on a quick call. Always looking to help with this sort of project.