a) Certified pre-owned and similar programs making some used cars more appealing (and thus more valuable), removing them from the supply of unvetted, cheap used cars.
b) More folks being able to afford used cars around the world. It seems like at the lower end of the market, many American used cars are shipped to Latin America or the Caribbean (in Europe many older cars are sent to Africa), again reducing the supply of used cars.
This is my sketchy recollection of things I've heard and read, I don't have any direct sources to link you to, but I've certainly heard multiple people bemoan that say, $5-10k doesn't buy you nearly as good a car as it used to (even adjusting for inflation).
EDIT: As an aside, the issue of shipping cars to different regions could also prove interesting for owners down the road. There are already some issues there - I believe my car's navigation system is for North America, or maybe US+Canada. So if my car gets shipped to Argentina in 10 years, the Nav will be useless or will need an update. Not a huge deal. But if Tesla's Autopilot is tuned for conditions and laws in a certain region, it would presumably require reprogramming for another region. Would that be an automatic over-the-air update? Or would Tesla not want to bother with free updates to a 20-year-old car?
The OP talking about a $5,000 car is talking about a 10-20 year old car with 100,000+ miles.
Both cars appeal to a different set of buyers. The CPO car effectively competes against new cars.
It's not uncommon at all today to see a car tick over 200,000+ miles, so buying a car for $5,000 with 100,000 miles on it can be seen as a pretty good deal. Usually people would recommend a good Japanese / Korean car (Toyota, Honda, etc.).
At the same time, emissions controls changed the market in that cars are more governed by use not age. I mostly buy Hondas and know the market a bit, and they are all good for about 200,000 miles without significant maintenance.
I sold a 2003 Honda Pilot with 250k miles for $2,500 last year -- really high considering all of the stuff that will break on a car of that vintage. In the 90's, that pricepoint would be for a much younger car. A place like CarMax will sell a 2010 Pilot with low mileage for $15k, which works because the TCO works out -- you won't have lots of maintenance.
That seemed to be true for a while, but I've seen prices relax quite a bit lately. It's possible this was just local, and I'm not seeing a broader trend.
It's also the case that perfectly good brand new cars can be had for $13-$14k, (Nissan Versa, Ford Fiesta) which I appreciate are not attainable for many people.
That's why you don't see less-wealthy people commuting to their jobs in an early 2000s Mercedes S-Klass or in a VW Phaeton (even though they'd be able to purchase such vehicles at today's prices), but instead they use an older Honda or a Toyota or a slightly newer Nissan.
Sourcing a CANBUS module for a 10+yo car is already a roll of the dice. Take all the modules that are required for the vehicle to operate, are matched to the VIN, can't be re-flashed, aren't available aftermarket and aren't made by the OEM anymore and anything that breaks where those sets intersect turns the vehicle into a brick. The list gets a little longer every day and won't get shorter until we get right to repair legislation with teeth.
You can drive a car without all the fancy electronic doodads. Everyone who drives a 20+yo car does it every day. You can't drive a vehicle that won't start because it doesn't see some quasi-essential (e.g. ABS) module talking on the bus.
I'm starting to doubt this more and more. Dumping the ROM from a read-only chip and flashing a new one isn't rocket science. After watching stuff like http://www.youtube.com/watch?v=yqWhId-tQXs&t=9m31s and guys like Dave Jones and Louis Rossman, it looks like the entire repair industry is long due for disruption. Sure, finding old or obsolete parts will still be a problem, but not impossible.
I suspect this will become a huge societal problem if it continues like this in a car-dependent country like the US. Yes, the middle-class people will probably still be able to purchase $30,000-40,000 cars for the foreseeable future (even adjusted for inflation), but what will happen to the people who won't be able to purchase a second-hand car that costs more than $5000-6000 (or even $1000-2000)? How will they continue to get to work in a country that values public transport so lowly?