A while ago I sold a few bumper stickers online and ended up using cafepress. I could have made a better profit by printing the stickers in bulk and mailing them to people, but I'd have to spend $100+ on paperwork just for the privilege of paying sales tax just in case I sell any to New Yorkers.
If small internet businesses had to pay taxes to the 40+ states that have sales tax plus to all the other jurisdictions (cites, counties, who knows what) in the U.S. it would be almost impossible to sell stuff and comply with the the law.
For AMZN the overhead is nothing.