This is a great viewpoint that is contrary to mine.
I don't personally believe that referral linking is anything detracting from a persons word or trustworthiness. If they are clear and up front about it.
Usually people just post links, but the way I find to be the best use of it is with transparency - including a caveat at the bottom that tells everyone about the fact that there may be kickbacks.
It doesn't cost the seller anything, and I see it as just another way of showing support. Does this make me immoral? Does it make my word worth less?

I guess it really is an individualized thing.