Would you buy a used torque wrench?
Maybe, it depends (Condition of the wrench, pricing, etc.)
My 1/2" torque wrench is a cheap 25 year old tool and is slipping occasionally.
Sounds like the ratchet mechanism? This could probably be rebuilt if the company is still around.
For the most part I just use it for wheels, which is fairly often.
Budget of around $200. Is a higher end used one worth getting?
At that budget, you could get a nice new Precision Instruments split beam and not need to look at used.
If ridden hard, I worry it will be out of wack.
Possibly, but most things can be calibrated. It should also be fairly obvious if it was treated roughly (dents, scrapes, dings, etc.).
There are certainly models or brands that cost more (like Snap-On) but most of that is just mark-up and brand name not because they necessarily work that much better. For example, Precision Instruments used to make all of the older Snap-On wrenches before they gained in-house production (CDI), yet market price between the two is nowhere close because of the name.
I believe split beam designs tend to stay accurate longer than twist-scale round handle ones since the components aren't under constant tension and the operator doesn't need to remember to set them back to 0 between use. That might be an advantage buying used. Or there is Gedore's Dremometer type that is supposed to be extremely resistant to abuse (but costs $$$).
At the end of the day though, even a cheap beat-up old wrench could probably be used for lug nuts/bolts as long as it is consistent and you can compare it to something else to know how much it is off by.