how does one bypass iCloud lock on MacBook

You don’t.

This feature was designed to secure your device from nefarious actors. If you could easily bypass this lock, it wouldn’t be of any value.

I bought a MacBook Air from someone and they didn't properly factory reset… so far I've tried to get in contact with the person to no success they no longer respond…

This MacBook Air is probably stolen. A legitimate seller would work with you to un-authorize the device from their iCloud account before selling. Even post sale, there would be no issue in responding to you - it’s a super simple process to sign out of a device.

any course of action other than selling for parts as its still in perfect working order.

Unfortunately, no. You’ve got an expensive paper weight on your hands now.

Going Forward…

This will be an expensive lesson to learn. However, going forward, use the checklist that Apple provides for transferring ownership as a “buyers’ checklist” of sorts. Don’t accept possession (don’t let money change hands) until these things are complete.

As for the device, you might be able to sell it for parts, but you could also report this to your local authorities. If you transferred money via app, there may be an electronic paper trail they could use to identify the sellers. I’m not saying this will be successful by any means, but you won’t know until you try.

Besides, if Find My is activated on that device, the original user could ping your location and you’d technically be in possession of stolen property. That’s not a good scenario to find yourself in.