Any business that sells a product by weight has to have its scales checked and calibrated at least annually. They are open to inspection by Trading Standards in what was weights and measures. All market scales that are using weight for selling stock by the kilo should have a sticker on the scales or near to them with the date of the last test. Some of the older markets that sell by the "head" only use scales as a guide so are not required to calibrate annually. However in some areas it was "tradition" to round down the weights. This is probably illegal if you are using calibrated scales. Some older scales that do not automatically tare could also be set to a lower starting weight for zero. This was acceptable when you were weighing product inside a container. You used the weight of the container as the tare. Most modern markets now use electronic scales with digital readouts so there should be no room for errors except where the booking clerk still uses a calculator to divide by the number of animals. I do understand there may be markets in the outer reaches of the country that may have a bent abacus. On the issue of weighing livestock and losing or gaining weight, any lamb weighed in the afternoon I would subtract around 2kg from the expected morning liveweight and cattle could be up to 20kg on grass. Obviously animals on ad lib feed will be more.