You'll need good luck to find a replacement active matrix LCD that doesn't have the problem.

It's a very common problem, not only for the Powerbook 170 but also for other models with an early active matrix LCD like the 180, duo 250, 280, 540,...

Your best option would be that you actually can test the screen before you buy the laptop. Leave it turned on with a white background for 2 hours to make sure.

Some panels show the problem in less than 10 minutes, others only after more than 1.5 hour.