Buying Online

[BladesNBarrels]

What Ray said.

Since 1937, CO law requires all residents to pay "use tax" on any goods purchased on which CO sales tax (actually the State allows a credit for Sales Tax paid in another jurisdiction) was not already paid. There is a special form (DR0252) for it and everything.

And, the State is now making it easier and more convenient by allowing an annual Use Tax filing along with your Income Tax filing.
Currently, all that is required is the 2.9% Use Tax on the State Form, but if you live in a Home-Rule City, you need to check with them regarding their Tax Regulations.
Jake Jabs has been leading an effort to reduce the number of jurisdictions that require separate filings for years.