As an undergrad, my chem professor told me that as a rule of thumb, drop the last decimal point from digital scales (as its worthless)
Meaning, if your scales are 100g +/- 0.1g then it should only be used for measuring whole grams. Further, if measuring points; get a digi that goes to .01 or .001. Even then, the build quality will determine how quickly it looses accuracy. Moreso, carrying it round in a pocket will quickly fuck it up - they are precision instruments after all.
The beams are great for accuratly pre-weighing retail bags etc. But a bit impractical whilst wheeling and dealing (jewellery of course..). It is important to have a set procedure for weighing and minimise external factors such as wind and ensure the surface on which you weigh is hard and flat. You can end up unknowingly ripping off the buyer or yourself. Also, it is wise to carry caliberation weights to handle any disputes if they arise.