Some believe that there is no such thing as a MINT card unless its still in the pack (thus the premiums on older wax packs). Following is a brief summary of card grades:
MT (Mint)- only if it is in perfect condition. Usually reserved for cards still in original packaging (unopened packs, etc) Many collectors consider most cards to actually be in NrMT because they have been handled to some extent.
Nr-MT (Near Mint)- may have one micro defect.
EX-MT (Excellent-Mint)- may have two or more micro defects. Value is 65 -80% of Nr-MT. A "Micro Defect" would be fuzzy corners, slight off-centering, printer's lines printer's spots, slightly out of focus, or slight loss of original gloss.
EX (Excellent) - has minor defects. Value is 50-65% of Nr-MT. A "Minor Defect" would be corner wear or slight rounding, off-centering, light crease on back, wax or gum stains on reverse, loss of original gloss, writing or tape marks on back, or rubber band marks.
VG (Very Good) - may have one major defect. Value is 30% of Nr-MT
G (Good) - may have two or more major defects. Value is 10% of Nr-MT. A "Major Defect" would be rounded corner(s), badly off centered, crease(s), deceptive trimming, deceptively retouched borders, pin hole, staple hole, incidental writing or tape marks on front, warping, water stains, or sun fading.
F (Fair) - may have one catastrophic defect. Value is ?? - depends on buyer. A "Catastrophic Defect" is the worst kind and would include badly rounded corner(s), mis-cutting, heavy crease(s), obvious trimming, punch hole, tack hole, tear(s) etc...
It is also worthy to mention that any sort of tape marks, pen marks, etc. make the card a DEFACED cards. An autograph on a card technically defaces the card. In most cases, selling the card would depend on what the buyer would be willing to pay. People who like autographed cards will likely pay a premium if the card is autographed.
Card centering is one of the most subjective criteria for grading a card. What one person calls 60/40 another may say 55/45. Here's an idea of what the different centering benchmarks mean.
Horizontal centering and vertical centering can be down the same way. Lets say the left border is 3 mm and the right border is 2 mm. This gives a total border of 5 mm. Divide 3/5 to get the % of left border 2/3 to get the % of right border. In this case, you get 60/40.
Centering is more difficult to judge on non-border cards. However, it helps if can can compare cards from the same set. If you look at several cards from the set, you can get an idea of where the player or team name should be located. When you are familiar with this it becomes easy to identify an off-centered card. You can also look at the backs of several cards and discern where the name bar or logo should be.
Of course, you're not going to carry a ruler around with you to card shops or shows. However, use this simple scale to determine condition.
centering 55/45 or 60/40 drop to NM-MT
centering 65/35 or 70/30 drop to NrMT
centering 75/25 or 80/20 drop to EX-MT
centering 85/15 or 90/10 drop to EX