A lot of sellers now include a rating figure in their item descriptions to indicate the condition of the item. Some of these (C) numbers can be confusing, especially as different sellers use differing scales.
I have found that the following scale is probably the most beneficial when purchasing the average figure. It is not designed to grade the top end figures, as these are more specialist and can have their own rating scale.
C10: Never handled and so is in PERFECT condition, will probably have been stored in the blister but has no card present.
C9.5: Near mint condition with tight limbs and perfect paint work.
C9: Near mint condition with tight limbs and near perfect paint work.
C8.5: Figure is in a very good condition, with reasonably tight limbs, but may have minor paint loss.
C8: A good figure with fairly tight limbs, suffering some paint loss.
C7.5: The average figure, possibly with a loose limb and some paint loss. (some sellers can determine this has PLAY WORN).
C7: More than one loose limb and more excessive paint loss.
BELOW 6.5: Normally these will be sold as spares and may have considerable damage, but this should be apparent in the item description.
REMEMBER THAT THESE GRADINGS CAN STILL BE VERY SUBJECTIVE AND WILL BE IN THE EYES OF THE SELLER!
The best advice is to buy from a reputable seller and also bear in mind the cost of the item bought, if you paying a very low price for a common item do not expect the world, but also if you are paying a great deal of money for a less than common item then you should get what you are paying for. You can always ask the seller questions before bidding, if you are not happy do not bid. If the item is not what you expected when it arrives then contact the seller and strive to find a solution together before leaving negative feedback. Sellers work with your buyers to provide a good customer service.