Un-registering the serial number is only part of the process.
The seller must also provide you his activated license on a USB-eLicenser dongle.
If the seller had purchased a boxed version, it came with the license already activated on a USB-eLicenser dongle.
If he had purchased a download version, he might have stored the license on a Soft-eLicenser (virtual dongle on the hard drive of his computer). In this case, he needs to purchase a Steinberg Key (or an Arturia or Vienna dongle, typical cost around $25), and transfer his license to the USB dongle.
More information regarding the eLicenser system is available here:
http://www.arturia.com/evolution/en/support/elicenser.htmlPlease note that for the transaction to be complete, you need to get both the activated eLicenser dongle and the serial number.
(if the serial number is still registered, Arturia can transfer the registration, but we will need confirmation from the seller to process the transfer request from the buyer)