I wouldn't put too much stock in Amazon's inventory numbers. They were at "2 Used & New" on Thursday and look, they found a box in a forgotten corner of a warehouse out in the remote reaches of cyberspace!
"New and Used" are Non-Amazon Sellers. So number of sellers for a given item can fluctuate constantly.
It is very much like ebay except that Amazon offers certain guarantees and acts (as much better IMHO) mediator between buyers and sellers. There listings are not as active as Ebay becuase their commissions (Amazon's) are much higher (to sellers). However you tend to get a MUCH better level of service when it comes to order/seller/buyer problems....should they occur.
Kinda get what you pay for type thing. Given two items on Ebay and Amazon, if both had the same price/condition, I would go with Amazon as they are more reputable and safer....for both buyer and seller. (Seller's becuase payment is done immediate through Amazon's credit card system and buyers becuase Amazon has an A-Z gaurantees)
The cost to the buyer would be the same, but the seller would pay more in commissions, which tends to atrract higher quality sellers and less problems.
Suffice to say, if you buy from either (Ebay or Amazon), you probably will be fine 99% of the time, but that 1% will be when going through Amazon will make all the difference in the world.
Just my two cents.