The seller should contact Support to transfer the Push serial number to the second-hand buyer. We also recommend including the original invoice or proof-of-purchase with the unit. This way, the new user will receive information about Push updates and be able to make a warranty claim, if the unit is still in warranty.
Ableton only issues a Live 9 Intro license or offers a discounted upgrade to Live 9 Standard/Suite upon first registration of new Push units (unless you already own an Intro, Standard or Suite license), so second-hand units are not eligible.
Any license included as part of the second-hand Push sale should be agreed privately between the two parties. If an Ableton Live license should be transferred, in addition to the Push serial number, it needs to be explicitly requested when the seller contacts Support.