One common way to grade you action figures is to use the C-scale. There are several different levels to the C-scale and it usually refers to the card more than the actual figure. When you are grading and your card receives a good grade it is more than likely that your action figure is going to be in perfect condition. (not always the case though)
Here I have provided a guide to the C-scale for your reference when buying action figures.
The card should be flat with no tears, bends, or marks. There will be no holes or stains on the card. The figure should appear as it was just coming off the production line. A figure in this condition is very hard to find and many people will never grade one with C10.
C8-C9 (Near Mint)
This figure should be close to perfect condition but there is room for a few flaws. The card should be near flat with bright colors, original gloss and no stains or tears. This is considered a hight quality collectors toy. C8 is usually the lowest collectors grade that collectors will pick up.
Some curling or creasing on the card is allowed. There may also be other apparent flaws such as yellowing bubble, minor tears, or fraying on the edge of the card (less than 1 inch length). There will be no cracking of the ink. These are collected for nostalgic value only.
The card can have permanent curling and creasing also allowing cracked ink. The card color will be flat though there will still be color. The card will have small tears (less than 1/2 inch), stains, writing, and small holes.
The card can be completely folded and creased permanently. The gloss on the card will be gone and the colors have faded and discolored.
C1 (Very Poor)
This is the lowest grade for a carded action figure. It can have any amount of damages but the action figure must be attached (doesn’t matter how little)
Some sellers will use the C-scale and I made this for your reference when buying online. You can also check out my article about AFA action figure grading.