Just in case what? If a used Lexus has received regular preventive maintenance and has not been driven aggressively and abusively, never run low on engine oil or coolant, etc. it will have little mechanical wear and tear and therefore the powertrain warranty is a waste of money. That's why it's important for used Lexus shoppers to get to know the original owner which is not possible if they purchase from a Lexus dealer.
Every car maker offers certified cars and extended warranties and all of them have "certified" cars that actually have considerable mechanical wear and tear that can come back to haunt the used car buyer soon after the extended warranty expires. That's because cars that have considerable mechanical wear and tear can still breeze through the certification process and because the certification process is limited in its ability to detect whether or not a car has been driven aggressively and abusively, never run low on engine oil or coolant, etc. Furthermore, no dealer or dealer sales rep will publicly admit all this which is one more reason to steer clear of dealers when shopping for a used Lexus.
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What makes you so sure you'll actually get to know the original owner and his/her driving habits? They'll just lie through their teeth to get you to fatten their wallet.