goto "your account" and select manage license next to all of the licenses that show your ignition key icon. i think there is a remove or de-authorize option -- i'd test it and post the actual steps, but i don't want to mess up my key
Here's the info on buying/selling used copies https://www.propellerheads.se/suppor...ion=buyandsell
I suggest you read the seller and buyer stuff so you know what to do from your end, and what the buyer is supposed to expect. note that once you find a buyer, it can take a few days for the transfer to go through - and it won't happen over a weekend, as it is done manually by someone at Props during sweden business hours.