I live in the UK, and been looking at the car trade quite a lot. So if you get an unregistered car from Japan with a low mileage, you can trust that since I've been there too and they do drive very little so mileage is very low (15k-30k normal i guess). BUT if you get a low mileage car grey imported from UK with really low mileage, check it properly as it could be tampered with and for me it smells fishy. Cars here are very high mileage as people travel a lot (motorways - highways are free in the UK). 3 years old car with 50-60k MILES is normal here. So be careful. Check the condition of the car matches with the apparent mileage. If it looks tired its has travelled a lot.
Just my two cents.