Vinyl records grading scale


I am sort of new to vinyl and am building up my LP collection. A very prime source for it is the internet used market where appears to be sort of a grading scale system to rate condition of vinyl records on sale (G+-, VG+-, EXC, NM, etc). Is there any place I can refer to for the criteria or guidelines as to what to expect from the different ratings?
Also, what has been your experience buying used, and what is the lowest acceptable rating to get substantially noise free (tic/pops) records? Please let me know, thanks.
jmr
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.
I guess that based on responses one would ideally stay on EX/VG++ or above unless there is something really special that you want and theprice is adequate.
My limited experience has been that you can aproximate the noise level of most records by visual grade on relatively clean records. I suspect most of the ratings you see on sale offers are made pretty much that way. Again, thanks for your comments.
In your experiences where are the better source of good sellers? Ebay? Agon? other forums? private sellers or dealers? Thanks.