Simple. If you had an Amazon and ebay link, you would put them in your sidebar and perhaps quietly plug them once a month.

You would not, if writing a post on British Airways, find a convoluted route to add in 20 links to books on BA on Amazon and 20 pieces of BA ebay memorabilia, because that would be stupid. You would stick with the sidebar link.

Swap 'Amazon and ebay' for 'credit cards' and you can see how to retain your credibility!