Brand new Amazon Fire Phones with a free 1-year of Amazon Prime can be found on eBay for around $120-140. They are sold unlocked and without a contract. Basically, these are all the Fire Phones that AT&T couldn't sell. Amazon Prime costs $99/year, and if you already subscribe to Amazon Prime, the free year will tack onto your existing subscription, meaning you won't have to pay for your next renewal. So if you're already an Amazon Prime user, the real cost of the Amazon Fire Phone will be $20-40 depending upon the deal you find. They are an amazing deal if you're in need of a very very good, but not flagship cellphone without a contract.
Yes, they can be activated on T-Mobile, Cricket, or any other GSM carrier in the USA or Canada.
If you find new Fire Phones without the free year of Amazon Prime being sold by individuals on eBay, then that person bought the phone at the $120-140 price, activated the free year of Amazon Prime that came with the phone, and are now selling the phones to offset their Amazon Prime renewal rates. If you do the numbers, you can end up with very cheap to free Amazon Prime renewals for many years to come doing this...
Yes, I own one via such a deal and I like it a lot, but I did not buy a bunch of them for resale purposes.