As other posts have mentioned - there is no known workaround to the lock and you will not be able to use that device until the account holding it locked releases the hold.
However, knowing that it's not stolen and having gotten the seller's word that they will unlock it - you do have a little leverage over the seller. You can identify them and hopefully you have some idea of their contact information.
Their Apple ID is likely their email address, so have you tried mustering as much kindness and sending a polite email reminding them how you are eager to make use of the phone they sold you. See if the phone will show you their Apple ID - unfortunately for you (and fortunately for people who have their phone stolen) - on most screens you can't get to the full text of the Apple ID without knowing the passcode.
If you've exhausted the gentle ways, you could try checking with a local lawyer to see if they in any way violated local laws for sale of goods under a false description. Your costs for a quick consult and how to take them to small claims court might result in eventual repayment of your costs and the purchase price.
The local police might also be able to think of ways to convince the seller to do the correct thing at this point.
It's hard to say, but you are really at the seller's mercy if you hand over money before verifying the phone is really unlocked and free of their hold. For others, this article has the steps Apple recommends before buying a used phone.
You could try appealing to them, but I would start locally rather than try to get Apple to break into someone's phone when they don't really know you or the seller.
Contact the pervious owner and ask them to log into iCloud themselves and release this phone.
Failing that, ask if work has this device enrolled in an MDM or to contact Apple to validate via the serial number that the device is not owned by a consumer or different company. In that case, your IT department can bypass this activation lock. If work has an HR department, they can request / require the previous owner to follow Apple's steps to release the device. Legal could bill the past employee for the replacement cost.
You probably need to just turn the device in or wait it out since bypassing the lock isn't viable.
Best Answer
Official steps to turn off Find My iPhone Activation Lock are documented by Apple at https://support.apple.com/en-us/HT201441
In a nutshell, the various ways to remove iCloud Activation Lock include:
For consumers, you will likely need to make an appointment at an Apple Store and provide the Genius on-hand proof of purchase (to show you're the authorized owner) along with proof of identity and they will reset Activation Lock, giving you access to the device.
For businesses, it depends if you are enrolled in DEP - Apple's Device Enrollment Program or just want to provide copies of the original purchase listing serial numbers to verify ownership.
If you bought the device second hand, stole it, found it or came to possess the device by some means other than buying it from Apple directly, then you will probably need the original owner to show proof of purchase, etc. They will then transfer the device to your account or simply disable Activation Lock.
The only self-service way to remove the lock if you can't type your password into the device itself is to authenticate at https://icloud.com and remove the activation lock when the device is not connected to the internet.