Vinyl records grading scale

I would say that ebay seller typically visual grade only and even then, its often in a pretty dim light. I would consider most records to be overgraded by one grade. VG+ really is VG, etc. and M- IS the lowest I would go. Most importantly, if you are paying a premium price, they should be play graded. Just my $0.02.

This is a very sticky and subjective issue. The most important thing, as Arahl indicates, is to know your seller. The good ones will honor your returns if the record is not up to grade. But, as Undertow, suggests, the difference between an excellent LP and a poor one may be a matter of your own cleaning.
There is no absolute answer to your question as it depends on what you are willing to pay etc. A VG+, pre digital, reissue LP for $20 may be a very good deal compared to spending $300 for a NM- version of the same record as an orignal issue,... or maybe not. It depends on what you want.
Welcome to the world of analog collecting. It requires a substantial investment of effort into understanding labels, quality of reissues, remasterings and searching for what you are interested in, otherwise I would recommend investing in digital.

by sticking with a more selective visual grade you reduce the risk of noise, though the risk is still there.

I agree that most record grading is spurious and has no correlation to the sound. Many records are M or near mint. The latter should mean pristine, perhaps played once and unmarked, they very seldom are. Visual grading is useless anyway, you need to play both sides and what commercial seller has time. I agree with Arahl, get to know sellers you can trust and stick with them.
A good vacume clean does make a big difference too.

From the following grading
Mint, NM, Ex, VG+, VG, Good, Fair, Poor, and Trash

If you have an expensive cartridge (over $300), which would be the least condition that you will be trying with your cartridge? How about for budget level (under $70)?

I have over 500 LPs, and most of them (about 300 that I bought from Garage sale at under $1.00) are Good ~ Trash condition. So far, I have not bothered play them on my Grado Black cartridges and inexpensive Shure cartridges. I recently purchased more expensive ones, and I wonder whether I'd rather be more careful.