A few questions concerning competition in results pages

Hi all

I am relatively new to internet marketing and am preparing to get my first websites up and running. I have a few questions about checking the backlinks of one's competition (I realize there are a host of other aspects that play into one's rankings). I am using Yahoo Site Explorer.

1. When checking the inlinks, what filters do you want selected? (I assume it would be "except from this domain" , but to the entire site or only that specific url?)

2. Why is there a difference in the results when including "www" in the url instead of just typing in the basic domain (e.g. yoursite.com)?

3. I am looking into marketing physical products, mainly from amazon. The first Google results page for prospective keywords usually contains a majority of online retailers (Amazon, etc.) as well as the company's webpage (if my keyword is that specific). These sites obviously have quite a bit of authority, yet the specific pages have a limited number of backlinks coming from outside sites (most come from pages within the same domain). I have heard people say to not go for the keyword if there are too many large retailers on the first page, yet I have also heard the opposite because of the low number of non self-referential backlinks. How do you approach these situations and which is preferable?

Thanks for reading. Any thoughts would be much appreciated.