Theoretically, as search engines and advertisers both get more and more sophisticated, the organic and paid for links should converge. The most 'relevant' results for (search term, searcher history, other relevant data) indicating someone looking to buy something will be the sellers most likely to convert the given searcher. At the same time, advertisers would be unwilling to pay for clicks from searches indicating unlikely buyers. If matching ever get that sophisticated, some altruistic sounding blurb about keeping commercial links off the organic section probably won't be far behind, however.