Never rely the seller for your own good. I am not saying that all sellers are cheater, but best rule of tub is whenever you get car YOU change yourself almost all oils and fluid and keep the count of kms and months.
It will help you track the exact detail, as may be old owner has wrong or forgotten details and while selling car nobody is very specific on telling you all the dates and km done on engine oil, gear oil, PSF, brake oil, coolant etc.