As a seasoned (although retired) pen-tester I wanted to say you'll have some serious problems with rating when it comes to results.
When a company hires pen-testers and pen-testers do or do not find stuff, the company has no idea about the coverage. So the pen-test team might have missed many stuff or identified all. I'm sure you've seen in real world even the same members of the same pen-test team might find different issues for the same test.
Therefore one of the biggest problems is to actually knowing whether they are good or not at what they do. It's easy to rate communication skills, responsiveness, attitude, report quality etc. But very hard to rate the quality of the results (which is the real reason for carrying out a pen-test).
When they don't find something, maybe there really is nothing there. When they found something, maybe there is more there. The customer has no idea at that point. It'll be only a fair amount of time later they'll figure out the coverage / vulnerability finding quality.
I'm sure in the long run market will stabilize (assuming you can change your rating for a pen-tester even after a year) but this is something to consider.
Update: BTW personally I don't like the idea of logging in via LinkedIn (for finding a security talent), it's feels too intrusive, beside of the personal preference my experience showed me especially security industry don't like SSO style things.