Instructions
Get a Paypal account. While many sellers will take a check or money order, some only accept Paypal. Having Paypal assures you of having the widest range of sellers to choose from on eBay. Additionally, using Paypal qualifies you for eBay's Buyer Protection, which can help you resolve a dispute with a seller if you don't receive the coin, or if the coin is different from the seller's description.
Know exactly what collectible coin you are looking for. The very least you should know is the denomination, type, year and mint of the coin you are seeking. It helps to have a preferred grade in mind because it will narrow your auction search parameters. If you are looking for collectible error coins, be familiar with common abbreviations used in the market, such as DDO and VAM. Having a specific coin in mind not only makes using the search functions easier, it will save you money by avoiding mistakes when reading the titles and descriptions of the auctions.
Be informed about the current price your collectible coin is going for outside of the eBay auction market. Check current coin magazines and price guides, available at newsstands or coin-dealer websites, to get the most up-to-date prices. You don't want to find a coin and think it's a great deal, only to find out that other collectible coin dealers are selling it for less. At the very least, you should have a copy of the current year's "A Guidebook of United States Coins" (aka The Red Book) by R. S. Yeoman. This is a general-purpose price guide and is the standard that others follow. Doing some research ahead of time will save you money and save you from making mistakes.
Limit your search to collectible coins that are certified by one of the top four coin-grading companies: PCGS, NGC, ICG and ANACS. Unless you are an expert, judging coins based on just the picture provided by the seller is extremely risky. Damage to a coin or signs of cleaning may not be noticeable in the picture, but can drastically reduce the coin's value.
Check the seller's eBay feedback rating. You should only buy from reliable sellers who have a proven track record of providing collectible coins with accurate descriptions and good customer service. If there is any negative feedback, read it and judge if the buyer or seller was at fault.
Check the seller's return policy. Just in case you do not receive the collectible coin you thought you would, being comfortable with the seller's return policy before bidding will make rectifying any mistakes easier.
Check shipping and insurance costs. Some sellers recover fees or pad their profits by charging higher-than-necessary shipping costs. Make sure you add these costs to your desired purchase price for the collectible coin.
Enter your bid. Find the maximum price that you feel comfortable with and don't exceed it. You don't want to get into a bidding war and end up overpaying for your coin! Choose an amount that is at or below the lowest price you have found in your price guide or other research.
Check the coin once you get it to make sure that it is exactly what you expected from the seller's eBay description. If not, immediately contact the seller to return it and make amends. If it is what you wanted, remember to give the seller some positive feedback. It's nice to help a reliable seller succeed and it could be the start of a mutually beneficial seller/buyer relationship for you both.