The correct solution is a no ads marketplace. Where search results return what the user searched for and literally no one (not even Amazon) gets to pay to put their products at the top of the results.
But SEO is quite hidden and hard to define. Ads are there, see a banner. SEO? No obvious 100% criteria for it.
Can one tell that a text is produced by SEO? Usually no (especially, when it's good quality).
Now, the difference is:
- In ads case, a government comes and asks for a tax. Because there is a clear case for a transaction in an economy sense (ad for money).
- In SEO case, no transaction proof is there. Can't come and ask to pay tax, or regulate anything. It could be a happy customer who wrote a piece of text. Or clever SEO author. Or both mixed. Can't tell! In this sense SEO brings less or no tax.
How do you define that? Do you exact match on query text? How products that convert better? Have lower return rates? Is a 1 start product that matches the query exactly better than a 5 start that mostly matches? Etc, etc.