Colo. Code Regs. 39-26-102(19)
"Wholesale sale" means all sales of tangible personal property or specified services to a licensed retail merchant, jobber, dealer, or wholesaler that purchases the property for resale, and all sales of tangible personal property or services specified in C.R.S. 1973, 39-26-104 to manufacturers and compounders that incorporate such tangible personal property or service into a substance, commodity or product for resale.
All sales to the user or consumer of tangible personal property or services specified in C.R.S. 1973, 39-26-104 are taxable retail sales, regardless of the purchaser's trade or business. A reporting form will be furnished annually to wholesalers to report such retail sales.
It is the duty of the vendor to collect the tax unless he is furnished with satisfactory proof that the sale is exempt by statute. Also it is the duty of the vendor to obtain the sales or store license number or other satisfactory proof if the purchase is for resale. In case of doubt, the department should be contacted or the tax collected.
It shall be presumed that any purchaser not having a valid sales tax license or store license is the ultimate user or consumer of any property that is purchased. Any sale to such a person will be a taxable retail sale regardless of the disposition of the property sold, unless the vendor can establish that the purchase was for resale in the ordinary course of the vendee's business.
39-26-102(19)