Maybe you could contact Steinberg support with the dongle serial number to see if it is valid?
If the license is on a dongle, that means that they activated it, so at the very least they had to use the Elicenser Control Center. Whether they used the software or not is immaterial because the installers are freely available to all, simply by downloading the Steinberg Download Assistant.
If the seller is claiming they have an unregistered license, that would be a red flag to me since It’s the dongle to which the license was downloaded that is registered to the owner’s account on Steinberg.net.
There were a series of ebay fraud sales where the seller sold either dongles or activation codes that were later found by the buyers to be not-upgrade-able due to the licenses having been originally acquired through some kind of fraud, credit card or otherwise. The dongles these license were on were flagged- and tranferring licenses off them, or updating to newer versions were blocked on those dongles. I don’t know much more than that, but there was some discussion in the forum about it.
A receipt for the purchase of the license would be a good idea, and there could be a separate receipt for the dongle. But if it were fake, how would you know? Only a receipt from the store Steinberg contracts with, Asknet.com, would be a sure thing, it seems to me. Maybe a Guitar Center or Thomanns UK receipt would be viable since they are giant resellers.