The best reason to keep a copy of your consignment agreement: If the gallery goes out of business through bankruptcy (and yes, even really good ones surprise us), you MUST have the proof or you WILL NOT get your art back. The courts will consider it property of the gallery unless you have proof that it's on consignment. I have a friend who lost thousands of dollars worth of art this way - and the gallery had been in business for over a decade. It was unexpected.