Grading has contributed signifcantly to the problems in the sports card hobby. I would not trust the grade on any card from any of the grading services including the ones you mention. There have been too many cases of cards being regraded numerous times in attempts to get higher grades. Yes, even by the so-called "better" services. Same card, numerous submissions to same company, numerous different grades. There was even a case of a card being replaced by the grading company for an inferior grade card. Slabbing is a cancer on any hobby as far as I am concerned.
Ebay is a relatively small part of the hobby. Do not base all your conclusions on what you see there. There is a much large picture to view.
Paul