I bought a 2nd hand S61 keyboard last week and was very pleasantly surprised that upon re-registration I got the same freebies that a buyer of a new keyboard gets: Komplete Select and a £44 voucher. Kudos to Native Instruments for their generosity.
I aso understand that when you sell hardware you can transfer the bundled software and keep a copy for yourself. Seems that NI is very keen to maintain resell values and that is a great policy.