Bought a refurbished MacBook Pro from eBay. Trying to make my iCloud account the default, but cannot sign out of previous owner's account.

the original owner is the only one who can help you - have them follow this procedure - What to do before you sell or give away your Mac - Apple Support

If they refuse then get your money back as you have purchased an unusable device (probably stolen) and you need to either get a refund of report the device and transition to the local police

LN

Try doing Internet Recovery, then erase and reformat the disk, and install the OS.

How to reinstall macOS - Apple Support

Activation Lock Problem

If you purchased the device second-hand, then it is useless without the Apple ID and password to which it is locked. If you don't have it, can't remember it or cannot get it, then the device is useless to you. Try to get your money refunded, if you can, or take it to an Apple store for an exchange provided they will help you. If you are the original owner then take the device, a copy of your purchase receipt and identity proof to an Apple retailer where they may be able to assist you.

Have the previous owner signout of his account for that device.

Not sure why my previous post got removed.

I did not say anything that would be considered a breach of the TOS.

Restoring the MacBook Pro, via the Recovery Console will remove the Apple ID and Password from the computer so it can be setup.

Activation lock does not apply to computers in that sense, and as such the Mac is not unusable as claimed.

You should be able to do that and not need the previous owner.