Did you call Janome or the Janome dealer? All you have to give them is the serial number and the warranty should transfer. They use the serial number to pull up the warranty info when servicing.
I would still buy it if it is a great buy. 90% of the service issues we had on the Janomes were because they were threaded wrong...either up top or in the bobbin. Customers would just get frustrated and have brain 'farts' when re-threading and usually they missed the loop on the needle bar, causing tension issues.