If the activation is asking for the owner's iCloud account, you will not be able to bypass it unless you happen to be the original owner with purchase receipt.
Your best bet is to contact the seller and have them give you their password so that you can log into the Mac and then removed the Apple ID / iCloud.
Turn off Activation Lock before you send your Mac in for service, sell it, or give it away
Before you send your Mac in for service, sell it, or give it away, you should turn off Activation Lock. If you have access to the device, turn off Find My Mac to disable Activation Lock, and then sign out of iCloud.
- Choose Apple menu > System Preferences.
- Click Apple ID, then click iCloud.
- Turn off Find My Mac.
- Click the Sign Out button.
If you can't access your Mac and you need to turn off Activation Lock, follow these steps.
- Go to iCloud.com and sign in with your Apple ID.
- Click Find iPhone.
- Select All Devices, then click your Mac.
- Click Erase Mac. Select Next until the device is erased.
- Click next to your Mac.
What to do if your Mac starts up to an Activate Mac window
If you've purchased a Mac from someone other than Apple or an authorized Apple reseller and the device starts up to an Activate Mac window, the device is still linked to the previous owner's account. You'll need to ask them to remove the device from their account.