The problem with grading is that it is an art and not a science. We have all seen grades that we disagree with and that have sometimes shocked us and it really doesn't matter whether it is the TPG doing the grading, an auction house, a dealer or an individual. I have had some good surprises and plenty of bad surprises.
My experience of NGC is that they have been pretty strict. My experience of LCGS is that they either overgrade or that their mapping to US grades is out by a couple of points. My experience of buying non-slabbed coins at auction is that they are often overgraded; "almost mint" is usually far from that. And there are plenty of dealers who overgrade too.
Ultimately, you have to buy the coin and not the grade. But you also have to understand that, if buying on the basis of a photo, that it can be 'doctored'. There is a lot of risk involved. Buying from a dealer in whom you have trust can substantially reduce the risk. And, despite the above, buying slabbed coins also reduces the risk as, for the most part, they tend to get it more or less right.