If I really wanted to bother buying a bike from the said dealer if it were me I'd want a copy of the green book after checking the frame number and engine number also the signed transfer DLT paperwork by the previous owner then take the copies to the DLT to check, if they say it's OK I would say it's OK to buy.
Once the transfer paperwork has been completed at the DLT office and the green book is in your name the previous owner can do nothing.
The money lender shops or pawn shops that give money against a bike will keep the green book if the bike owner doesn't pay then the lender takes possession of the bike and then get their money back buy selling to a secondhand dealer, hence cheap.
Whether the process was done properly of getting the owner to sign the bike off on the DLT paperwork is fake or not, the fact is in the possession of the green book.