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.
(1) A packet of yeast is typically 7g exactly. So if you're buying yeast by the packet, use one packet. But assuming you will be measuring from bulk yeast, the correct measure by volume would be 2 1/4 tsp instead of 2 1/2 tsp.
(2) The weight of salt relative to its volume varies considerably depending on what salt you are using - the best way to work out the appropriate conversion for the salt you have on hand is to consult the Nutrition Facts on the back of the container. Most salt gives the nutritional information based on 1/4 tsp. - but the weight of this amount of salt depends on the type of salt and size of grain.
Here's how you do the math.
Start with the information on the back of your package - for example, let's use my Fine Mediterranean Sea Salt - 1/4 tsp. = 1.2 g.
Multiply both sides by 4 to get the weight of 1 tsp. - 1 tsp. = 4.8 g - i.e. 4.8 g. per tsp.
You need 10 g - so divide 10 g by 4.8 to get the number of tsp. you should use - 10 / 4.8 = a little more than 2 tsp.
You really should get your measurement this way instead of relying on a generic conversion formula. There is significant variability from salt-to-salt. Another salt I use has a label that states 1/4 tsp. = .7 g (the crystals are larger so the same 1/4 tsp. now weighs much less than the sea salt).
1/4 tsp. = .7 g. (multiply both sides by 4)
1 tsp. = 2.8 g.
So this salt is 2.8 g. per tsp.
You need 10 g - 10 / 2.8 = a little more than 3.5 tsp.
Best Answer
Although I've come to find a lot of uses for my kitchen scale, the ones that immediately pop into mind are:
When buying, you want to think about the following:
As far as price goes, I think $20 is too cheap, and is likely to become unreliable quickly, if not immediately. That said, I think over $100 is too much as well, that's likely to be based on design or name. My current scale cost me about $50 and works great, and I check its calibration regularly and haven't had a problem yet.
One specific example from our house - I make smoothies every morning from 20 oz frozen fruit, 10 oz soy milk, 6 oz greek yogurt. My wife is following a very specific nutrition guide right now, so I then split it into 12oz for her, 24oz for me. Getting her nutritional info right without a scale would be all guesswork (and I've come to learn, through using my scale, that my guesswork used to be pretty far off - it's much better since I started using the scale regularly).