Some used car dealer roll back odometer?

Yes it is; you can make a $1000 or more. Depends where you live but here, the mileage is declared every time a vehicle changes hands making it easier to spot any abuses.

They used to. They used to disconnect the cable that ran the odometer and attach an electric drill to it. Maybe they cant do that now.

They used to, but the majority are digital now, or have an MOT record that can be traced

Used to be fairly easy, and I expect many smaller dealers actually did because it increased the car's desirability. It used to be that white lines would show between the numbers if they rolled them back, so the dealer just rolled it ahead until the desirable number showed. Not so easy with modern electronics, but it can still be done. And just as illegal as ever.

It IS illegal and a small number of crooks do it because:

They can sell the car for more
They probably won't get caught

Buy from a reputable dealer and check the vehicle history - service records and such.

No history - no buy-ee

They used to. All you meed to do to verify the mileage is to look at the last emission test. They list the mileage on the form.

They did before digital odometers and the Internet where it's easy to check a vehicle's history and previous odometer readings (e.g. Carfax and AutoCheck).

No. Federal crime. Plus it is not that easy.

It is possible. I heard you remove the instrument cluster and send it somewhere overseas where they specialize in this. They mail it back to you with a lower reading. Beware, federal law does not apply to vehicles over ten years old.

In the UK it's easy to check a vehicle's mileage back to when it was three years old.
Any vehicle less than three years old should have a stamped service book, if not walk away.

Some might take that gamble. It is a Federal Crime punishable by orange jumpsuit with a hole in the rear.
I had 2 cars where the odometer stopped counting after 60K miles. Getting a new "clock" was costly so I took it out of my car and on the kitchen table FIXED IT. A Manufacturing defect. Did not care to pay several hundred dollars for a new one, which would turn the odometer to ZERO. So car lots would not go out and buy odometers just to replace other ones.
Most people do not pay attention to it. If the car looks new inside and out then the mileage should be low.
If car is beaten up, the mileage is high.
You can figure it out by many other indicators.