Colo. Rev. Stat. § 18-16-109
The provisions of this article shall not apply to private collectors purchasing collectors' items from other private collectors or businesses engaged in selling valuable articles exclusively as collectors' items, and who pay for such purchases by check, nor shall the provisions of sections 18-16-101 to 18-16-108 apply to valuable articles purchased exclusively in interstate commerce and paid for by check mailed to the seller in another state, if a record of the check by which payment was made and the name and address of the seller is maintained for a period of three years, or a retail merchant who, in a retail transaction involving the sale of a valuable article, receives another valuable article as a trade-in and credits the retail purchaser with the value thereof if the retail purchaser provides proof satisfactory to the retailer that the valuable article was originally purchased from that retailer. For the purpose of this section, a "private collector" is an individual, business, or corporation who purchases an item for a price based on the value of the article as a historical item rather than the prevailing market price of the item's metallic or stone composition; who has an interest in preserving the item in its unique or historical form and who does not alter the form of the article; and whose primary purpose is to keep the article in a collection or to sell to another collector.
C.R.S. § 18-16-109