This thread may be interesting to you: Cross grading experiment to PSA
Personally I would not trust that a lot of those 3rd party companies 9/10 actually will transfer over to a 9/10 at PSA or BGS. However, there are certainly some hidden gems to be found if you look at the card itself, not just the grade. Overall I bet you’d have a better chance to profit just off buying raw cards and grading them.