I read about weighing coffee to the 0.5 gram, but what's the margin of error for a typical 5kg/11lb kitchen scale?
Every scale should indicate its accuracy in its manual.
A good guess if you have no manual is that the accuracy is approximately half of the size of the smallest unit it reports (for example, my digital scale reports down to grams, so its accuracy is probably within about 1/2 gram). If the scale is analog, you can get an idea by the smallest interval on the dial or indicator.
Still, only the manual will tell you for sure.
Scales that go up to 5 kg are unlikely to be good for measuring 0.5 gram quantities like coffee. You will want a scale specialized for that level of quantity.
The exception is laboratory quality scales, which have higher accuracies and greater accurate ranges, but they are not pretty and are expensive.
Mine gets it right to the nearest gram.
It's easy enough to test the accuracy of your scale using water. For tiny increments, you can use coins. A US quarter weighs 5.67 grams, a nickel weighs 5.0 grams, a dime weighs 2.268 grams. I'm pleased with mine. It doesn't do fractions of a gram, but when I weigh 2 quarters it comes up 11 grams occasionally bouncing up to 12 grams. I can't imagine any kitchen application (outside of molecular gastronomy) that needs more accuracy than that. My scale is "typical" as it has a capacity of 5 kilos.
EDIT: I've been meaning to do this anyway, the following will demonstrate just how accurate kitchen scales can (and should) be. This will also soon be a rave review on Amazon, especially since I only paid $16 for the scale.
Big pan of cans
OK, 4.585 Kilograms, pushing max capacity
Tare (weight - 0 grams)
OK, I said before that a US nickel weighs exactly 5 grams, right?
One nickel: (weight - 5 grams)
Two nickels: (weight - 10 grams)
Three nickels, four nickels, five nickels, six nickels: (15g, 20g, 25g, 30g)
My scale can beat up your measuring cup!