. . . . Are Magix users allowed to sell their Magix serial numbers to other users on this forum . . . .
You need to contact Magix Sales - to inform Magix them you want to sell the product.
If they agree it will be deactivated and removed it from your account and cannot be offered for sale before that has been done and you have receive confirmation.
There is no Buy & Sell section in the forum - you would have to offer the product somewhere else.