I recently sold something on Ebay for $100 that could be purchased for $106.99 on Amazon. I was surprised by how much it went for. The product I sold was new in the box, so there was nothing wrong with it, but I expected it to close for considerably less than $7 less than what someone could buy it for on Amazon. In fact, if they qualified for free shipping on Amazon, the cost differential would have been only $2.00.

This made me wonder: how much cheaper would something have to be on Ebay to justify buying it from a private seller vs. buying directly from a well known reputable online retailer like Amazon? This isn't to say that Ebay sellers aren't reputable. The site wouldn't be as successful as it is if they weren't. But there is certainly more risk tied into buying something from a relatively unknown retailer than buying from the biggest retailer on the web.

Put another way, what's the premium you'd be willing to pay to buy directly from Amazon over the discount you may be able to find on Amazon.

Another example: The Canon SD800 IS digital camera current costs $306.48 directly from Amazon. The same camera in new in box condition has been selling in the $240 range on Ebay over the past few days. Does the $66 difference justify buying from a private seller?

In my case, I would have gone with Amazon over myself for a $7 difference on a $100 purchase, and would have gone with the Ebay seller to save $66.

What is your break point, and how do you justify it?