Rule number one: You can't know everything. Still, you should try to learn the essential information about items that you list for sale. Some inexperienced sellers lose big money because they haven't bothered to find out if an item is rare. Other new sellers blindly accept someone's assurance that an item is valuable, and then try to sell it as a valuable collectible — until knowledgeable bidders let them know it's junk. Rule number two: Always take the time and effort to know what you have. Your long-term reputation as an online seller will depend on the work habits you develop starting now.

Auction site search tools and advanced search tools are excellent for discovering if there really is a market for the dusty yo-yo you've stored since childhood or for that ugly “heirloom” candy dish left to you by your aunt. As you move deeper into the auction business and start gathering goods from garage sales, thrift stores, store closings, and other sources, you will rely heavily on auction site search tools, as well as on Internet search engines such as Google.com.

You may be an expert in a particular category, such as antique furniture or sports trading cards, and will want to focus your auction business there. Yet even with an extensive background in one category, you must be willing to spend time seeking out new items, verifying their authenticity, listing them for auction, and keeping up with the markets for your specialized goods.