So when you can't even buy one new, why would someone want a used one? (unless, like I said, it's a pleasure driver or some exotic, which is a tiny fraction of all cars sold)
Think of all the things that have been replaced and superseded in history - once the new thing exists, the old one drops in price dramatically.
There are plenty of 20 year old vehicles out there with plenty of life left in them.
If people would want to buy them before considering availability of new ones, banning new one increases market clearing price of used ones by eliminating an obvious alternative.
Now, as the number on the road dwindles, you'll start to see reduced support infrastructure (gas stations, parts availability, etc.) which will make ownership less attractive, but that's likely to be a slow and lagging process. So, if there are still utility advantages to gas cars, I'd expect used ones to become more expensive initially with a ban on new ones, but drop sometime later.
For the same exact reasons many people today choose to buy used gas cars over new electric cars.
Because the prices of non-collectable ICE cars is bound to plummet unless we haven't solved the battery charge speed issue (as in, in practice, supposedly it was recently solved in theory).
>because laws will ban the sale of the "old tech".
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