In order to bid favorably in any auction, you need to know the market value of the item you are bidding on. Typically this takes time and research and comparison-shopping. Guidebooks are also incredibly handy.
We did a search on Amazon for coin guidebooks, which returned a result: A Guide Book of United States Coins 2007 (Guide Book of United States Coins) (Guide Book of United States Coins) by R. S. Yeoman and Kenneth Bressett (Paperback - April 30, 2006).
This book was priced at under $10 and was in stock. If you want to look for comic books, get Overstreet’s Price Guide, which has been around for many years and is widely regarded by the comic book industry as the standard.