I am doing my CO state taxes and the question is about online purchases where CO tax may not have been paid. How would I know the answer to this? I do not keep track

[MichaelL1]

If you do a lot of on line purchases where sales tax was NOT paid, you can estimate this. 

Many on-line retailer's now collect sales tax I am finding out, or at least where I am. 

[clsheehy]

Some do but not at the correct rate.  At most, they are probably using the state tax rate only, not any special taxes that apply like city, county, regional.  I'm curious about how anyone can estimate this - they would need to keep all receipts for all purchases and check to ensure that rate taxed (if any tax was applied) is the correct amount.  I'm guessing the majority of people have to idea what the tax rate is for where they live or how to find it!

[MichaelL1]

I fully agree with everything you are saying.  A few states started adding this section to their tax returns and now every state with sales tax seems to, as it brings in revenue to the States.  I have never heard of a state checking this area,