Is my iPhone 6 useless now? What do I do?

I just bought a gold 64GB Sprint iPhone 6 from someone on Facebook. Before buying it, I looked up the IMEI on both Swappa and CheckESNFree. It popped up as clean. We met up twice because she didn't have the box, case, and accessories during the first meet; she was very chill. She even had kids. There was no activation lock or anything. After a day of messing around with it and trying to activate it, I couldn't because it was still activated under her account. She talked to a Sprint rep and they said it wasn't activated under her account. After restarting the phone, there was Sprint LTE services and incoming voicemails. But from her number, her account. I've turned off the data (because I'm probably using hers) and tried contacting her, but she hasn't replied? What can I do? Have I been scammed?

Probably a scam, and quite a good one. But all hope has not been lost. Just sell it to some other unsuspecting person for $10 higher than the price it was sold to you. This way you not only get rid of a completely useless phone, but get your money back plus $10. Be sure to block all communications with this person to avoid any trouble. Or if you really want to turn this into some cash, find a partner(lets call them X) Sell X the phone for $10 higher than you bought it for. X sells the phone to someone(lets call them S) for $10 higher than you sold it to him. You have both made $10. X and you buy a pack of 5(the brand) gum using your profits. X gives you his pack. You find S, get him to bring up the phone in conversation, bargain the price down to $10 lower than S bought it for (use both packs of gum as bargaining material). Buy the phone, at its original price, and give S both packs of gum. Find another victim(lets call him S2). Sell S2 the phone for $10 higher than the original price. Split the ten between you and X. X and you again buy a pack of 5(the brand) gum. Give your pack to X. X does to S2 what you did to S. Repeat for infinite profit.

You're not scammed. Sprint has lied to you and the phone is still under her account partially. You can try factory resetting the phone but don't think that'll help. Are you trying to use the phone on Sprint? If then you can request Sprint to activate the phone on your account and if they say that the phone is still locked to some other's account, you'll know. If you're not trying to use the phone on Sprint, factory unlock it and that shall solve the problem. If you need more info on getting the phone factory unlocked or activating on Sprint, mail me.

Try to do a hard reset. If you still have problem visit the link below to find the solution:
http://jailbreaksiphones.com/iphone-disabled/how-to-factory-reset-any-disabled-or-password-locked-iphone-65s5c54s43gs-or-ipad/

You need to go, to the sprint store with her and have them transfer the phone from her account to yours