If you're collecting coins which don't cost $1000 apiece, then
ebay is a Godsend. It's the world's largest coin store, and every price is negotiable. At any given time, there are over 100,000
US coins available.
The dark side to that optimistic statement is, if you are not an informed and disciplined buyer, you are going to get taken. Misrepresentations and shady sellers abound. There are many instances of bidders getting caught up in a frenzy, and bidding a coin to well over its' true worth. As a buyer, you
must know the coins you're looking at - grading, value, rarity. You
must have the discipline to walk away from a coin which isn't shown in sufficient detail. You
must have enough knowledge of coin photography and post-processing software to know when a picture has been doctored, and it really helps to be able to recognize a good coin which has been poorly pictured. You
must be willing to set a firm price that you're willing to pay, and walk away if that price is exceeded - there will always be another coin, on another day. It's been argued from a moral standpoint for years, but the only way to consistently get the coin you want at the price you're willing to pay is by sniping it.
If you can develop the skills and discipline, then
ebay is your greatest asset as a collector. The rules can be defined very simply:
1) Don't shop there for coins worth more than, say, $200 unless you're a real expert.
2) Know the coin you're buying.
3) Know the potential misrepresentations.
4) Research the seller.
5) Set your price and stick to it.
I try to follow those rules as strictly as possible, and generally I have good luck. Even so, I've been taken a couple times. Of course, I've also nabbed some real treasures.
Caveat emptor.