Coin grading services provide a niche service that cannot and should not be done by amateur coin collectors. It takes professional coin graders years of training and practice to note what makes a mint coin mint, and a lesser-graded coin less than mint. There are several subtleties in this field, and several reasons why you should not attempt coin grading services yourself.
Your Level of Expertise
Even if you have been collecting coins for years, that does not mean you have the necessary expertise to grade coins. Collecting coins and grading coins are related, but very different, fields. Without coin grading expertise, you could mistakenly lessen or increase the value of a coin that is either less or greater in value than the grade you give it. People could lose money buying and selling incorrectly graded coins.
Loss of Money
A coin you grade higher sells for more money. If the coin is not that good, the buyer loses money buying the coin and you could be charged with fraud. You could also be sued for the return of the money when the coin is properly graded and found to be wanting.
Conversely, if you make a mistake grading a coin at a lesser value, only to find later that the coin was much better than you graded it, you would profit from your mistake. The seller would lose money after having sold you the "worthless" coin, and again you could be charged with fraud. You could also be charged with theft, since the coin is more valuable than you said.
Tiny Details You Could Miss
Some coins are so close to one grade or another. The final decision is in the details. Is the mint mark perfectly stamped and raised? Are the edges of the coin completely without blemish or wear? Are there any tiny scratches on the face and tail of the coin? These are the details coin graders use to narrow down the possibility of a coin having a higher or lower grade than during the initial inspection. Even the pros have to use a jeweler's glass (monocle) to get up-close views of all edges and sides of a coin to decipher the exact grade of the coin.
Take Your Coins to a Professional
Always take your coins to a professional coin grader. if you want to become a professional coin grader, it will take a few years for you to get the basics down. Even then, you will have to consult a professional to see how spot-on your grading is. When a pro verifies that you are consistent and accurate in your grading, then you can grade your own coins and other people's coins.