Prior to agreeing to anything, Ask for the XenForo license validation key. This can be obtained in the customer area of the seller's XenForo account. In addition, the validation token, also ask for the domain associated with the license. This is to ensure they just didn't find the validation token somewhere on the internet. Once you have those 2 pieces of information, complete the following form and it will return the status of the license: License Validation API | XenForo