METRO PCS Sells Useless Phones Clearwater Florida

In 2013 I purchased a Samsung Galaxy S3 from the Metro PCS store in the mall. The phone costed me over 400 dollars new, which to me is a lot of money. I used the phone for about a year and then stoped using it because I had to downgrade to a cheaper service and phone. About a month ago I decided to put the phone on Craigslist and sell it since I dont use it, and I needed the money. I found someone who wanted it and she came to buy it. I was happy. Until she called me demanding her money back saying that Metro PCS told her that the phone was no longer usable on thier service. Apparantly Metro changed from CDMA to GSM. So I took her money back and got the phone back. I couldn't beleive it. So I went into one of the corporate offices today in disbeleif asking questions. They basically told me that the phone is now useless with any carrier. So I bought a 400 dollar paper wieght. This just inferiates me. I dont have a lot of money and live paycheck to paycheck and I really feel as if I was ripped off.