From my point of view the dealer was at fault and this fiasco should never have happened. When I bought my current van, the seller was not the registered owner at CRIS. It's easy to check - I just gave CRIS the name and address of the seller and CRIS told me that the seller was not the currently registered owner. They won't tell you who is registered for "data protection" reasons, but at least it alerts you to a potential issue.
When this potential issue arose for us, we asked for proof of the seller's identity and address, and made the sale conditional on successful transfer of CRIS registration. If the dealer taking the 'van had made those checks and obliged the previous owner to properly complete the CRIS registration, the issue wouldn't have arisen.