Lets say I have a domain already, for example www.automobile4u.com (not mine), with a website fully running and all.

The title of my "Website" says:

  <title>Used cars - buy and sell your used cars here</title>

Also, lets say I have fully SEO the website so when people searching for the term buy used cars, I end up on the second or first page.

Now, I want to end up higher, so I go to the google adwords page where you can check how many searches are made on specific terms.

Lets say the term "used cars" has 20 million searches each month.

Here comes the question, could I just go and buy that domain with the search terms adress, in this case www.usedcars.com and make a redirect to my original page, and this way when people search for "used cars", my newly bought domain name comes up redirecting people to my original website (www.automobile4u.com)?

The reason I believe this benefits me is because it seems search engines first of all check website adresses matching the search, so the query "used cars" would automatically bring www.usedcars.com to the first result right?

What are the downsides for this?

I already know about google spiders not liking redirects, but there are many methods of redirecting...

Is this a good idea generally?