I would not hesitate to buy used if there is a history on the machine, all original accessories are included (do double check against the new one), if there was a reasonable discount over new, and if the dealer is reputable. Given they mentioned a used one coming in, they sounds pretty up front to me, but as a direct customer, you can tell better.
Any warranty would have to be dealer-only (Janome, as with almost all brands, does not transfer warranties to second owners).