HTC One M9 Sprint Please HELP

Hello, I have bought an HTC One M9 Sprint on eBay from an US Seller, he told me that the phone is unlocked for international use. However, today I got the package and I tried to put my Sim Card... but it doesn't work, it says "Invalid Sim Card in phone" and "SIMLock Enabled" and even it doesn't ask for a code.

Please HELP ! I can't afford losing my money and keep an useless phone :(