Rookie mistakes amaze me! When buying coins the first question i always ask after careful study is do the laws of supply and demand have much control here? Second question i ask is the deal too good to be true? If it is it probably isn't
Why would you buy a coin without some type of serious knowledge of what you are recieving? I won't say great deals do not abound because truthfully some people don't know what they have, in the same breath, most usually have researched the value intensely. Buying lots that advertise multiple "deals" on fly by night auctions simply amaze me. First the advertisements promise the world vaguely, they often use pictures of not the actual coin's and even tell you that much. If you cannot see the coin how can you establish value? As a smart collector/investor before you buy a coin you should become somewhat knowledgeable about what it is you are buying. It will hone your senses to know approximately what something is truly worth if you know what it is, the condition, and rarity. Then you should consider popularity and desirability.
By no means am I the smartest buyer but, I know what I know and I have usually researched before I buy. Good luck it's a shark eat guppy world.