[nhdev]

As someone who has run a couple sites like this I can give some insight to that.

You don't necessarily need permission to display an offer but you do need a relationship to get a commission on the sale. These relationships can either be brokered through so called "Affiliate Networks" (most common) or when a site gets big enough they could be direct business development deals with the commerce site's marketing team.

It is a low profit business segment as it is (you have to generate a lot of volume to make serious money) so diluting it by including non-commission websites is not very common.

It is, however, somewhat common to list higher commission products near the top of the list. This is a poor user experience so usually it is only done if the two offers have the same price.