It's already in the license agreement under "Rights"
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.
I understand the why .. which is a valid point to the xF developers but :
Let say I own more than one license ... and If I decide to sell the site I can't just hand over my account since it's tied to other licenses as well ... Hmmmm .... catch 22
I always thought the 120 days was to discourage transfer fraud. Charge backs following transfers can be a big problem. In that sense the restriction protects potential second hand license buyers. As for the "no third hand transfer" thing... I can testify to the fact that repeated transfers can get really messy from a record keeping and enforcement perspective. Plus the risk of fraud and piracy goes up.
I am just guessing as to the reasons for the transfer policy. I think the restrictions make sense. In fact, restrictions like these are not uncommon for electronically distributed software like xenForo.