I'm a coin collector.
Gold and silver coins have two things that go into their value: Precious metal value and a collectors premium. Collectors premium is the condition of the coin, its rarity, the year etc. It all comes down to how rare it is. The only way you can make money is if you can get another collector to pay more than you did. So in that case you NEVER buy them at auctions, ebay auctions, brokers etc. You try to get them from someone who is eager to sell. So I would not deal with collector coins unless you knew exactly how to buy a coin WAY below its valued price.
So all in all buying gold and silver coins all comes down to playing with TWO market variables: Collectors value AND metal value. So it is way more tricky than just buying metal.
I'm currently looking into buying copper pennies. Why? because the price of copper is not that far away from the face value of a penny. So worst case scenario you can keep your principal of one penny =).