Let's break this down.
BRAND NEW:
pros: Its brand new! everything is guaranteed to be fresh, new and original - Agreed
0 mileage - goes without saying
no hidden defects or anything....UNTRUE. How many brand new recalled Toyotas have you seen this year? New cars break down all the time. Have you ever been to a Lexus dealer service department? They are just as busy as the Chevy dealer or CARMAX.
cons: we have to face depreciation, which sucks - True but that isn't all. You pay way more for the car, You pay for it longer (loan terms are generally longer) and your insurance is more expensive
SECOND HAND:
pros: relatively cheap compared to a brand new one. - TRUE but that isn't all. You can get a great used car with a warranty that will last you just as long as a new car if you shop smart. The car will look just as good, ride just as good and last just as long.
cons: you'll never know whats hidden underneath _ UNTRUE - If you do your research and you inspect the car you'll end up with very few if any "hidden" flaws.
been used already - Goes without saying but that isn't necessarily a negative.
sellers say lies - UNTRUE assumption. You can't assume that every person selling a car is a liar. There are thousands of good people selling good cars. You just have to be a good judge of character. There is alos no guarantee that the new car salesman is being truthful either.
different parts from unknown sources - What does that mean? Are you assuming that parts you buy from Autozone are somehow all inferior and should not be sold? That is a moronic statement. I worked at a GM dealer for a decade and if we didn't have a part in stock we went righ to the NAPA store to buy one. The parts were manufactured in the same factory and put in a differnt box depending on who they were selling them to.
A used car is a smart financial choice. If you care about driving a good car and saving money and not superficial things like "it's been used" then a quality used car is the best choice. But in the end it's your money.