While disputes will rarely happen, your reputation may rely on your ability to prove your items are authentic. For example, many stones are often marketed as “turquoise” when they are in fact manufactured from other stones, dyed or in some other way not really natural turquoise. This is understood in the industry, but it may call into question the authenticity of genuine turquoise. Having a receipt can help you prove to a customer or, in some cases, a payment processor, that you are being honest.
An example of when this can affect you financially is if a customer buys a piece of jewelry from you and pays with PayPal. If they believe what you sold them is not genuine, they can file a claim through PayPal, and your only recourse is to provide proof that you purchased genuine materials.
In seven years of making and selling jewelry, I have never had to answer a dispute like this, but I have peace of mind in knowing that I can prove the authenticity of anything I sell. Keeping receipts will protect your reputation and your business.