I bought a used iphone 6 and it wants a 6 digit code that the seller doesn't know. how do I get into my phone?

You don't, return it and get your money back. There is no way to bypass the Activation Lock, If he doesn't know the code to his own phone, it is most likely stolen.

You won't be able to, if you know the seller go and return it to him and get your money back else this phone will be a brick!

Absolutely agree!