Every used car for sale is being sold for a reason; either there is something wrong with it or the seller thinks something is about to go wrong with it.
Many cars are sold simply because owners want a newer car. This is often based on the mileage of the car, or because the car has one or two small issues the owner doesn't want to fix.
We have purchased many used cars over the years, all of which have been at least 15 years old. Sure, there's usually a few items that need fixed, maybe new tires or brakes, but you'll run into that with new cars as well.
I purchased my current 2000 VW Jetta back in Jan 2017 for $1900. I've spent a bit over $3000 on various parts since then (tires, brakes, new struts, various small items like door locks and tune up items). My total cost for 2.5 years is only $5200. Barring an accident or major failure, I see no reason why I can't drive it a few more years.
We bought my wife's 2004 VW Jetta in Oct 2018 for $3500. Other than tires and a new windshield we've hardly done anything to it. Total cost so far is $4900 include the car, taxes, licensing, and tires.
My daughter bought a 2003 VW Jetta in August 2016 for $3900. Her total cost so far is $5800, including the car, tires, brakes, and misc items.
Every car is going to need maintenance or basic wear items (tires, brakes, struts, etc.) at some point, whether you bought it new or used. I would never waste money on a vehicle that will lose value before I can even pay it off. That's a bad investment.