If new car prices go up, used car prices will go up - so if tariffs affect new car prices, then yes, the used market will be affected.
I almost bought a new car last year but only wanted to buy it all cash and ended up doing something else with a chunk of the money. I had a small fender bender this year which was my fault. No insurance involved as the other guy was good to go. Anyway, I was debating fixing the hood or not, and then I looked at new car and used car prices. They were insane, including the used car prices.
I ended spending 3K on fixing my car cosmetically and some worn out parts that were not defective, just end of life, and two new front tires. I have a pre-2010 Honda Accord with 150K miles and have had it for life, so I know it's history of maintenance and issues - and it's rock solid mechanically. I was looking at cars 10-20K - even at 10K I wouldn't have gotten much of an upgrade from my current vehicle.
If you definitely will need a used car within the next year and can't fix up your current ride, look now. It might be slow during the holidays, look that up, and if so, like start looking tomorrow and take advantage before Trump does his thing and while you might be able to get better deals at this time of year as well.