If memory serves it usually wasn’t that dramatic but there were significant outliers where it was 50+% cheaper and I don’t think I ever saw it go significantly the other way so there was a strong inertial tendency to just buy it on Amazon rather than put time into shopping around.

One other neat thing was international availability — Amazon.co.uk had Terry Pratchett books months ahead of the US release, so I could read them at the same time when people on Usenet were.