I dunno. When my wife is buying stuff over the internet the weights are given in grams and not Newtons.
No, the masses are given in grams (or kilograms), by definition. In a zero-G environment things weigh nothing, but their masses do not change.
Yes, that is very common, but it is still incorrect. There is no harm done generally unless you try to do something like a physics calculation.
Warning! Physics equations follow!! For example, you may remember F = ma, which says that the total force acting on a mass allows you to calculate the acceleration of the mass. Force is in newtons, mass in kilograms, and acceleration in m/s^2. A newton therefore is also a kg-m/s^2. So, you put the mass in as kilograms to get a force in the calculation. Weight is a force, so it must already be in newtons. As I said, it usually doesn't matter. In the equations above, though, it makes a big difference.