In Colorado, sales taxes are collected only on the first sale of a vehicle. After that it's registration taxes only, which are based on the car's depreciated value from new.
Be glad you live in a state where the sales tax laws make logical sense. Some states have decided sales taxes are just a form of revenue, and that they don't have to make sense. So they tax used car sales as well. This creates the contradictory situation where two cars can be taxed differently, even though they're used the same amount (driven the exact same number of miles in the same number of years).
Car A is bought for $40,000. $2000 paid in sales tax (5%).
Used for 5 years and 75,000 miles.
Total sales taxes paid = $2000.
Identical car B is bought for $40,000. $2000 paid in sales tax (5%).
Used for 1 year and 15,000 miles, sold for $30,000. $1500 paid in sales tax.
Used for 1 year and 15,000 miles, sold for $25,000. $1250 paid in sales tax.
Used for 1 year and 15,000 miles, sold for $20,000. $1000 paid in sales tax.
Used for 1 year and 15,000 miles, sold for $15,000. $750 paid in sales tax.
Used for 1 year and 15,000 miles. Total 5 years and 75,000 miles.
Total sales taxes paid = $6500.
The states also like to call it a use tax instead of sales tax, so they can also tax out-of-state sales without running afoul of the U.S. Constitution. But if you think about it, the logical way to apply a use tax to used car sales is:
Car B is bought for $40,000. $2000 paid in use tax (5%).
Used for 1 year and 15,000 miles, sold for $30,000. $1500 paid in use tax. 1st owner only got $10,000 worth of use out of car, so he really should've paid just $500 in use tax. So he gets a refund of $1500 use tax.
Used for 1 year and 15,000 miles, sold for $25,000. $1250 paid in use tax. $1250 use tax refunded to 2nd owner.
Used for 1 year and 15,000 miles, sold for $20,000. $1000 paid in use tax. $1000 use tax refunded to 3rd owner.
Used for 1 year and 15,000 miles, sold for $15,000. $750 paid in use tax. $750 use tax refunded to 4th owner.
Used for 1 year and 15,000 miles. Total 5 years and 75,000 miles.
Total use taxes paid = $2000.
So logically, taxing used car sales should result in no net tax revenue, so the simpler solution is to not do it. But in any contest between logic and more tax revenue, logic loses.