You can try reading back V5 & V6 of this thread for the CBU Mazda 3.

As for the price I think it should be at least 25% depreciation from the new price for a 18 months old car. But the new price should not be from the CBU new price, it should be from CKD new price. As the CBU spec is between GL and HS the CKD reference price should be the average of GL and HS.

CKD ref price = (105 + 120)/2 = 112,500
Used CBU car price = 75% x 112,500 = RM84,300

IMO this should be the fair price. Of course some owners want to base their depreciation on the price they paid but when there is a cheaper CKD version they should take a haircut.

Ask to see all his service records including the service booklet and invoices to see what has been done. If there are no major repairs should be OK.

I'm not sure whether you can go to the Mazda SC where he does his service and ask them for the full record but it's worth a try.

18 months is very soon to sell a car. What is is reason for selling?

There is car inspection service available from Goo Kantei