Hey @PoliceFan! Philippe here. I hope you're doing well. 
Anyone can sell the phone they own, even if they're still financing it. The seller would still be responsible for paying the financing on their end. In short, there would be no problems if you wanted to purchase it.
That all said, I do recommend you fully test the device to make sure it does work properly and get some proof of the sale to protect yourself. Buyer beware after all!
I hope this helps.
