Why digital camera price varies among various online stores?

I have an idea of buying a digital camera online. Canon A710 IS costs $260+ in Amazon, $310 in Buys.com, $340 in Circuit City etc.
Why is this huge difference in price?
Does Amazon sells used cameras?
Which online store is reliable?

Always check www.bhphoto.com first, to have a reliable benchmark on what the price should be for new, US warranty cameras.

a few are biding websites and not buying stores
chk buying stores they might more or less be equal with minute variations

you will notice that Best Buy and Circuit City have very similar price - in the case of bestBuy because of the volume they purchase in North America (dont forget they are in Canada to , and own Canadian store Future Shop) they can usually have the best price.

Amazon.com does not stock any product and are able to have bottom prices as they simply purchase it from another supplier (possibly BestBuy or Circtui city) to fill your order.

I believe it has something to do with:
1. How competitive the stores are.
2. The volume of merchandise the store take from the suppliers.
3. The cost to sell the product.
4. The reputation of the store.