@Martin > Sorry about that - unfortunately the rekordbox license is not tied to the unit itself; the key number on the hardware's serial number sticker is for internal tracking, not to unlock software.
If the previous owner did not wish to sell the rekordbox license, they are within their right to sell only the hardware without it - but if they have indicated in the sale that the rekordbox license was to be included, you'll have to contact them and ask for them to file a ticket here requesting the license be transferred to you.
If they will not respond or do not wish to transfer the license, we are not able to force them to do so, as they are still able to use it with other products. You are able to purchase a license or monthly subscription at rekordbox.com.
Should you have any other questions, please let us know.