@RHRead The problem is that Magix support is currently very hard to reach.
Here are many reports that they currently don't respond.
You definitely need support to free up all licenses in your customer account. I suggest you try to get in contact with them before selling anything, otherwise you might not be able to transfer the software because you can't reach them.
While the EULA's for the software allows such sale/transfer, after notifying Magix of the intent to resell the product and de-activation of the products from your Magix account, Community rule no. 5 ie.
... Use of the community for commercial purposes of any kind is forbidden...
is quite explicit regarding advertising of the sale in the Community forum.
There are many software 'Market places' for such transactions.