That is true. Many of the sellers are really dumb. Am talking from personal experience.
Anyways, in that case your only bet would be to try and find maybe a list of serial numbers of Jap N3DS which might give you a hint of what the firmware might be, but even in that case you have the same dilemma where the seller would be too dumb to even check the serial number and send you the right one.
Edit: Also, I remember reading here yesterday where one of the GBAtemp member was actually in Japan, and he said he was having a very hard time finding a Jap N3DS on or below 9.2. So that's the current situation IN JAPAN. Not sure what that Amazon seller has. If he's like a local seller, may be you can ask his phone number and call him?