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.