Q: how to disable previous ipod owner's find my ipod?

You have to get the previous owner to do this:

Turn off Find My iPhone Activation Lock - Apple Support

Apple will not help.

If you can't contact the previous owner/seller this it is likely the device was lost or stolen.  In that case turn it into the police. The only use to your is a poor flashlight using the light fro the Activation screen

Not possible, contact the previous owner or if you can't, throw it away.