As the seller is the current owner of the vehicle, they would have covered the road tax for their respective country. 

Once you win an auction, and the vehicle is transported to yourself, you are expected to cover any road tax expenses as per your countries rules and regulations. 

To collect the vehicle and drive it back yourself, you need to make arrangements accordingly. 

It's however highly recommended that you get the vehicle thoroughly tested by a reputable garage before driving it for the first time. 

For any additional information, we would encourage you to contact your local authority on the matter.