That's certainly a possibility however there are few problems.
You couldn't review something you received as a gift. I suppose you might be able to get round this if it was something that someone had bought for you off your wish list but it's not ideal.
Amazon isn't always the cheapest. I've put a review on Amazon for something (laser printer) I bought elsewhere because the place I got it from was cheaper (special offer) and didn't have a functioning review system (or they've never published the one I wrote).
