Note: Amazon, unlike other search engines, ranks search results not just by “keyword relevance” but also by sales rankings. What does that mean? Well, Amazon wants to sell more books so they have a part of their search results algorithm that calculates how many copies your book has sold and compares it to others in the search results.
What are you basing this statement on?
It's based on my own experience with my books sales/search results and countless hours spent researching search results on Amazon. Every major competitive keyword is dominated by books with high sales ranks. I shall edit that statement, however, and let everyone know it's just my opinion and I have no way of knowing if that's true but I have found that simply selling more books through advertising or some other medium has improved my books search rankings on Amazon.
This is my experience, too. The more one of my books sells, the higher it appears in the relevance rankings. I think this is not true for freebies, but I can't say from experience since I don't do Select.
And yes, keywords and tags are very important! I use Google's keyword tool in addition to Amazon's search, like you say, and sometimes check out searches in forums and other sites. In fact, I do this before I start writing to help make sure I'm including everything I should be in the outline.
However, Amazon is phasing out tagging. I, for one, Cannot add or click on anyone's book tags for the last several weeks. I contacted them and was told tags are being eliminated. Why it has only started with a few, I don't know, but it seems kind of unfair.
You "shall"? I notice more than a couple of people talk like this in this forum and it's just...so weird and off-putting because it sounds like the way people talked in 1912, but it's almost 2015. I don't see this anywhere else online but here, and I was just wondering why people here talk like that. I was just curious. It's been vexing me for a little while now because I keep seeing this way of speaking pop up a lot here.