1) Little or no knowledge about the area in which they are collecting. This ranges from lack of even basic knowledge about identification, authenticity, history, commonly known fakes on the market, reprints, understanding of the material and real pricing (Experienced collectors know that the price in a price guide is usually much higher than what it will sell for normally). It also includes lack of basic knowledge about who are quality sellers and the respected experts, who are the sellers and authenticators to avoid, what is the recommended reading and resources. Even experienced collectors make mistakes when they go into areas outside their expertise.
While a beginner can buy inexpensive things (good fun and great way to learn), he should not buy an expensive item until he is experienced and has done lots of research. Remember that ALL BEGINNERS WILL MAKE MISTAKES, whether it's misidentifying, misdating, buying a fake or paying too much. A beginner buying a $5 counterfeit is a good learning experience, and paying $10 for a $2 items is no big deal. A beginner buying a $10,000 counterfeit or $7,000 for a $100 item is a disaster.
2) They do not do the necessary homework on a particular item before bidding or buying. Do you know how many people say to me "Do you know what this is and how much it is worth?" AFTER they bought the fake, instead of before? (Answer: lots). Proper homework includes looking into the seller, looking into general pricing, checking the sellers return policy and authenticity guarantee, using a reference book or website or price guide to help make a judgment about authenticity, identity, rarity and price. Proper homework includes going to a collector or dealer and saying, Do you know how rare this is?, Does this look legitimate to you? or Does this price seem okay? ... Again, there's no great harm placing a spontaneous bid on a $5 or $8 item you have no familiarity with or from a seller you've never heard over before. It's a different story when you are not doing homework before you place a $2,000 bid.
3) They do not use common sense with judging a sale or auction. By common sense I mean the most basic questions and logic that even someone who had never collected before should use. Below are examples of some common sense questions and logic the collector might say to herself.
"You know, I know nothing about this area of collecting. Id be pretty dumb to place a $22,000 bid, at least not until I learn more."
"I have no idea what this type of thing is worth. I should find out what is a reasonable price range before I bid $2,000. For all I know it could be worth $25."
"This autograph's letter of authenticity comes from someone I have never heard of before. How do I know if this authenticators any good?"
"This is fishy. A T206 Honus Wagner in that grade is worth at least $100,000. Why would this guy be trying to sell it to me for only $30,000?"
"This seller's return and authenticity guarantee is 'There is no guarantee of authenticity. All sales are final with no exceptions.' Doesn't this mean that if he sells me a forgery I can't return it? I think Ill find a seller with a better guarantee."
You would think that some guy trying to sell me a $30,000 Babe Ruth autographed bat would send me pictures that were actually in focus.
I see a lot of different brands of trading card graders. Are there brands that are more reliable than others?
I hear there are tons of autograph forgeries out there. How do I know if this autograph is real?