Why are verizon and sprint Iphones so cheap on eBay?

I don't know why but they are only about $50 or something, is there any limit?

Verizon and Sprint use a technology called CDMA, which means phones sold by Sprint and Verizon can only be used by those networks. GSM phones like the ones by AT&T and T-Mobile use a technology that can't only be used by them, but with any other GSM carrier in the world. In other words, I can take aGSM phone to Surprised and use it there by just popping in a SIM card, you can't do that with a CDMA phone. Because CDMA phones are restricted by what carried they can be used on, they are lower in demand do are cheaper.