I'm currently looking at yahoo adwords myself. my ebay store's main product, if you search on yahoo by brand name, I'm in the #3 spot if you spell it correctly, #2 spot if you use most common misspelling. So, no worries there.
BUT, if you add product type to brand name, I'm on page 2. And, if you search solely by product type, I'm way deep in the results (looked through 10 pages and couldnt find me). A big part of that is because of lots of competition on same product line. And, of course, there's simply getting it optimized to show up in top results.
So, for product line i'm considering adwords. That way will at least get in the top in the advertised section. Dunno how cost-effective it will be, but worth a try.
For yahoo adwords, there's a good beginners tutorial on it at http://www.ebayuniversity.com/features/ebay_live_2006/?ssPageName=CMDV:AB# Select the Pay-Per-Click Success video (plus tells you how to see which keywords people are using when searching for your stuff and even tells you how many searches were conducted on those keywords in the past month!)
Of course, Google is where it's really at, but can use some of the knowledge gained in video for Google too. Only thing about google is they wont tell you what your competitors bid on keywords. (whereas yahoo will). For Google, you can get a general idea what people are searching for by using this tool https://adwords.google.com/select/KeywordToolExternal?defaultView=2 and selecting "Global Search Volume Trends" in the dropdown box