Already selling it?
I think the big boys behind XenForo decided to put a time limit to prevent people from buying several licenses during the pre sale discount and then selling them at higher price a few weeks later.
Not that is your case but that is why they have imposed the time limit.
I think your best bet is to contact Ashely Kier or Mike directly.
As has been stated already, according to the License Agreement, you are not permitted to sell a license until 120 days after purchase:
The Software may be permanently transferred by the original purchaser to a third party ("Subsequent Licensee") not before 120 days following purchase, provided that the expiry date of the License falls after the date of the transfer. The original licensee may not retain a copy or installation of the Software. The Subsequent Licensee must agree to the terms of this Agreement and waive the right to transfer.