I would certainly want a good explanation of why a very-nearly-new car was being sold, and indeed I think rather few used cars are very nearly new. For a car that's at least a couple of years old, though, there seems little need for extreme suspicion, and so far as I know (note: I have no statistics on this) most used cars are not horribly defective, suggesting that hugely reduced price relative to new cars can't be mostly about the risk of such defects. (If the used car is half the price of the new one, it would need something close to a 50% chance of being completely worthless for defect risk to justify the price difference. Somewhat less than that, of course, on account of inconvenience and the fact that an unknown defect is worse than a known one, but I think not hugely less.)

I suspect (with no evidence beyond casual observation) that it's mostly a combination of:

Risk aversion. If you buy a new car, you can be pretty confident that it's in good working order (and have recourse if it turns out not to be). If you buy a used car, usually it's fine but there's a nonzero chance that you get screwed.

It seems to me that most people are too risk-averse, in the sense that if they were able to disvalue risk less then (1) most would be happier overall and (2) aggregate happiness would be greater. If you are less risk-averse than most, you might value used cars higher relative to new ones.

Laziness. Reducing the risk of buying a lemon requires effort (and maybe getting help from someone else more expert).

If you don't value your time at hundreds or thousands of dollars per hour, you should probably not see this as a compelling reason for buying a new car.

Signalling. If your car is intended (on some level) as a signal of what a nice car you can afford to drive, then a shiny new one is a better signal than a used one.

If you care less about signalling, or want to signal other things than willingness to spend, you might value used cars higher relative to new ones.

Sheer attraction to shininess. (Broadly understood.) New cars look nice. Buyers who don't think explicitly about how much they value such things may be willing to pay many thousands of pounds/dollars/euros for this.

My guess is that scarcely anyone would defend this after considering carefully what they're actually paying all that money for. (You can buy a lot of other nice shiny things for the difference in price between a new car and a merely newish one.)

You will gather from the spin on my comments that I am firmly in the "buy 'em used" camp. If you are more risk-averse than I am, value your time much more than I do mine, are keen to signal wealth, and find shiny cars especially attractive, then you may very reasonably make a different decision.