And possibly a sale tax exemption certificate. In some cases they are one and the same. Becoming a dealer is easy, becoming a successful dealer is much harder, and becoming a TPG grader is harder still.
I applied to NGC and PCGS for a grading position and never heard back from either, but heritage picked me up so its cool
Not only will you need a business certificate and resale certificate (easiest part of getting a business), you will also need to have a Brick and mortar shop. The shop is the hardest part of the game and also weeds out all the small time guys that would just pay 50ish dollars to try and get a dealers discount. Im sure exceptions are made but for them to waive the B&M part you better have one heck of a reputation! As far as improving your skills at grading, many helpful things already have been mentioned. I will say that a degree in something is always a plus but hands on experience is the key. Focus on a series and stay with it till you become efficient at grading it. Then after that move to another and another ect. I personally like to specialize in a certain area instead of going all over the place. Many major services like ngc, pcgs ect have graders who specialize in a certain coin series.
I don't know about other States, but in Arkansas, where I live, you also have to be a registered precious metals dealer with the Arkansas State Police if you handle bullion in a shop setting. And security systems! We won't even go there.
You don't have to have a B&M shop to be a dealer. I've known many dealers who operate online and then just work shows on the weekends. Sometimes just working the show buying a selling to dealers and sometimes taking a table to deal with the public as well. Working the shows and constantly rolling the money back into the business is the best way for a smaller guy with not a lot of capital to build his business. (Of course you will have to have an outside job to live on while you build your coin business because you will not be able to pull money out of the business to pay yourself for a long time. In fact at first for while you will probably have to put money INTO the business because in the beginning you will probably not be able to make enough on sales to both pay expenses and replace stock.)
I think State Law controls whether you need to have a license or certificate and State tax and reporting requirements so find out what is required in YOUR STATE. Of course Federal Law controls the Federal tax requirements on reporting and paying the Federal portion of taxes as well as the form of ownership you choose, ie, LLP, Proprietorship, General Partnership, Corporation, etc.
I think dealers start out as collectors. Most can grade to some standards in their opinions and can price coins with their own opinions. Some are self taught, some have been apprentices for years with other dealers, and some simply are driven and attend seminars and continualy study numismatics. No coin dealer is perfect or "knows it all"