How to unlock activation lock without previous owner?

You cannot. You'll need the owner to unlock it or the device is useless. (https://support.apple.com/en-us/HT201441)

the owner sold it to me on face book and never took off i phone locked to owner

You'll have to get in touch with the person who sold you the device, or recycle it responsibly.

I bought on Facebook ,I asked him but he blocked my Facebook.

Noway to contact the person

Then you should responsibly recycle your device. There is nothing you can do.