Why do people who are selling iPhone 4s sell them less if they are Sprint or Verizon?

I'm looking for a White iPhone 4 or 4s on AT&T. I was on Ebay.com and people that are selling the iPhone 4's for AT&T cost from $300-$500. But for Sprint and Verizon iPhone 4 they are in the range of $180-$250. Why is there a reason for this. And I also see the same thing on craigslist.org also.

Those other companies are less common when it comes to Iphones.