Amazon does not buy ideas. Amazon acquires companies with proven results.

You will need to execute on your idea, gain traction with users, build revenue, and prove that it is a fit for Amazon. Then, they will come to *you.*

If you want to have the partnership conversation with them early in your product lifecycle, make sure you're already hitting targets that are aligned with them and can show that you will enhance Amazon's existing offerings in some major way.

Here are some they purchased outright:

imdb.com
woot.com
smallparts ( http://www.amazonsupply.com/)
quidsi
diapers.com
soap.com
pets.com
audible.com
createspace
fabric.com
lexcycle
dpreview.com
alexa
A9
lovefilm.com
zappos.com
cdnow
shelfari
buyvip
askville
snaptell
tastemakers
customflix
Mobipocket.com
Exchange.com
bookpages

And here are ones they made partial investments in:

LivingSocial (invested $175M between 2010-11)
drugstore.com (owned ~46% of company in 1999)
Songza
Animoto
Foodista
Bill Me Later
Wikia
delicious