You're certainly welcome to purchase any of the partner services on your own workify site. You might have a couple reasons for wanting to do this:
Help marketing your workify site:
You can purchase any of our services if you'd like to utilize them with help marketing your workify site. Let's say you want to build up a social media profile, create Facebook Ads, etc. Just purchase and enjoy the boost!
Re-selling to your clients directly:
If you'd like to up-sell the services to your clients (since you cannot adjust the price), or bundle some services and sell a package to your clients, no problem. What you would do is purchase the service (or set of services) yourself, work with the worker to complete the project, and then re-sell to your client off-site.
For any of the services on your site that you purchase, you'll receive a portion of the purchase price back -- just as you'd receive if one of your customers purchased themselves.