OK, I am ranked one in an averagely competitive keyword but I see my targeted keyword is not a buying keyword. For example, I sell diet pills then I am ranked one in keyword "diet pills". But people who search for "diet pills" are not always the same with people who want to BUY "diet pills". Understand my illustration? I get plenty of visitors from a non-buying keyword but my conversion rate for my affiliate marketing is not good enough (from 250-300 organic visitors from Google per day, I only got around 1 sale per 2 days). Of course this is far from my expectation but I see that the other webmasters in my niche are also experiencing the same problem. If they chase non-buying keywords (although it promises many traffic), their conversion rate will be poor or even much worse than mine. But the dilemma is, if I chase a buying keyword (like "buy xxxxx", or "cheap xxxxx", the traffic will be much lesser.) Can I know what kind of keywords which are the best type for monetization?
You should be trying to rank for as many relevant keywords as possible. Keywords like "cheap diet pills" "discount diet pills" should attract visitors more likely to buy.
Another thing to consider is how you can monetize the non-buying traffic. Those visitors may still be interested in Google AdSense ads or related affiliate program ads. Since you are already #1 for 'diet pills', it should be relatively easy to also rank well for related action keywords. Just starting get more backlinks using each action phrase and the rankings should come in no time at all.
most of people want to buy diet pill will search for diet pill. When I want to buy something, I never use the buy keyword
That is a good point goodmast3r. Although people aren't actually typing the word buy, cheap, sale, purchase, discount, etc, a lot of them do want to buy. You just wouldn't get the same conversion rates with general keywords. Total sales/revenue is much more important than conversion rates. So be happy with your 'diet pills' ranking and keep working to add more keyword rankings.
goodmast3r and vansterdam took the words right out of my mouth. Don't be too concerned with buying keywords etc.. If you have first page positioning for your main keywords and phrases then your goal should be to keep that positing in the search engines, and gain more.