Talk to the dealer and check if he specializes with what you want to focus on
Say you want to focus on a specific kind of coin or a certain era when specific coins were popular. Maybe you want to focus more on gold coins and avoid silver ones. It is very important to see if the coin dealer actually specializes in that area and if he has the skill, knowledge and connections to procure the best possible coins that can help diversify your collection. Be aware that since there are practically hundreds of coins that have been made in the last couple of centuries or so, it would be difficult to find a dealer that can promise you that they will give all these coins based on your needs. It is advantageous to ask the dealers upfront and ask them if they can help you in expanding your collection effectively.
The numismatic community is pretty small and therefore, highly experienced experts with good credentials are necessary. Credentials and good reputation within the community can be hard to achieve. It takes several years to become a seasoned dealer for different coin variants. There are some qualifications that you can take into consideration when looking for a coin dealer. First, membership to notable organizations one popular group would have to be the PNG or Professional Numismatist Guild. This group is very strict with their requirements so being part of the group means a lot.
Does the dealer have enough funds or financial stability to keep their business in solid ground? It is ideal to consider this aspect since you want to make sure that your dealer will be around for a long time and that they can offer re-compensation if necessary. Financial stability nowadays is a big factor since you want a business that you can really enjoy dealing with to grow your collection. The confidence and trustworthiness of a coin dealer is not just with how good he is. Sometimes, there are non-business, personal factors that are also included in the mix so be sure that your dealer has the stability and good business practice to last a long time.
Is the dealer well connected? How is he connected to the industry? Has he been established for more than 5 years? Has he written a book or helped revise books or references? Is he well connected to different auction houses and other networks where he could acquire some coins that you might need for your collection. Especially for auctions, a dealer should always be on the know about everything happening in the industry. Without the knowledge and connections, how will he be considered as a reliable dealer? A dealer should have good sales practices and offer information to clients that are first hand information and not just hearsay and unverified information.
A dealer does not work half heartedly. He understands that each coin is worth something no matter how small or big it may be. A good dealer understands that your collection means to you and that you do not want to be ripped off by someone who thinks he could get away by offering a value just on face value. The dealer should be able to offer well researched and well grounded appraisals. You want to deal with someone who is reliable and can provide good service.