It can be a number of things from dodgy wiring, to faulty ABS sensor, to faulty ABS ECU, pump, any combination of the above, you name it...

A lot of people have ABS problems with their old sierras, common problem. Best get a brake specialist to check out the system for you, they should be able to give you an idea of what is wrong.

If the current owner will let you do this, you may then be able to get the price down depending on results.

Not sure if/how you can turn the ABS off altogether.

Good luck and welcome on board.
Greg