Certified scales are required when weighing any product and selling it directly to the customer based on the weight provided.  In the United States, you need to look for NTEP approval with a valid certificate of conformance number. Department of Weights and Measures officials can perform audits on weighing devices to ensure that they are legal for trade with proper inspections. Scales are required to be NTEP approved in all types of industries including food service, jewelry retailing and pharmaceutical compounding.  Farm stands, supermarkets or delis selling meat or fruit and vegetables by the pound must make sure their scale is NTEP certified and put into service by someone licensed in the state of operation before use.

You should look for the NTEP certificate of conformance to ensure the scale you are looking to purchase has been approved by weights and measures officials and has passed their testing process.  You still need to get your commercial scale inspected prior to use and can contact your regulatory authority for local and state requirements.  Each state can have different regulations and you always want to make sure you are abiding by any rules set forth.  One such rule can include requiring a dual display so the customer can read the weight on the scale.  In the case of a jewelry balance you may need to buy an auxiliary display that can be positioned in clear view of the customer making the purchase.

One popular function of a certified retail scale is price computing.  In the USA, you never want to purchase a price computing scale that is not legal for trade since it would be illegal to ever use.  Type approved balances will have a CC number on them which can be used to retrieve the official document on the National Conference on Weights and Measures website describing the specific model you own or are planning to purchase.  This document will note whether it is NTEP approved for a particular unit of measure and whether the counting feature is legal for trade since most often it will not be with the exception of prescription balances.  

An NTEP scale can have weighing specifications of 2,100 g x 0.01 g but only be certified to weigh at 2,100 g x 0.1 g.  It is important to be aware of this before you purchase a balance to ensure the scale meets the absolute precision your industry requires. Once the balance is sealed by the technician, it will be locked in NTEP mode and you won't be able to access the Non-NTEP resolution without breaking the seal.