For more detailed tips on search engine strategies, I highly recommend Search Engine Watch. It is the best resource on the net for learning about search engines.
One of your fellow users, Stephen Sherman, pointed out an interesting subtlety about keyphrase selection. Let's assume you find a keyphrase that you think people will type in a lot. Try it, and look at the results. If the results seem to be "on topic", then people are likely to drill down several pages to find a listing that is just right for them. This kind of keyphrase is one you want to target, but if you don't get on page one, you'll still get traffic. If, on the other hand, the results are mostly irrelevant (or full of spammed listings), then people will rarely look at page 2, or even more than the first few listings. These keyphrases are thus not as valuable. This doesn't automatically mean you shouldn't try to target it -- none of the criteria are absolutes -- but it does mean that it will be more difficult to get a useful listing with that keyphrase.
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