Instructions
Know the coins you're going to buy--not simply which ones, but what the common issues with each coin are. If it's an especially rare coin, you should find out about the coin that you plan on purchasing. Also, find out which district your coin was minted in, and where it's most likely to be found.
Talk to the dealer to know what kind of a person you're dealing with. If he seems especially trustworthy, probe closer with questions specific to the coin. Truly rare coins can be damaged by mailing them. Arrange a face-to-face meeting as you prepare to purchase.
Look at the usual coin characteristics, such as the wear on the head and tail. This gives you some general idea as to what the condition is. If the coin dates back to 700 B.C. expect it to have serious wear.
Haggle to get the best deal possible.