You can ask the seller for the license number. Then go to XenForo 'contact us' in the footer, and open a ticket requesting if the license is eligible for a transfer.
To not scam the seller, I recommend people to pay 50% up front.
And to not get scammed by a seller, I recommend buyers to only pay 50% up front and give the other 50% when you can login to your own customer area.
The seller's license must be eligible for sale, and when you get confirmation from XenForo Limited that it is, and you paid 50% upfront, the seller can open a ticket requesting to transfer it to you.
You can give the seller your name + email, and I believe that should be enough for XenForo Limited to create a customer account for you with the license.
The seller has to stop running XenForo and remove the files, etc.
As the new owner you can edit the license to point it to the domain where you will run XenForo.
You will have access to the 1.0 and 1.1 releases.
@wicked: I do not know what site he comes from, but my site XenFans has a marketplace where people sell/trade/buy vBulletin/XenForo/etc licenses.
As a licensed customer you can download any free add-on from this web site and/or third party sites.
Any paid add-on that didn't come included with the sale of the xenforo license will require to be acquired.