Both Buyers and Sellers can give feedback with 1 Star being the worst and 5 Stars being the best).  You can also add a sentence providing more information on your Feedback.  To do this go to My Stuff  >  Orders   -

There you can you can provide Feedback for all of your Orders (when it says Can't leave Feedback then the item has not been sent yet).

Please keep in mind that it's important to leave feedback only for what is in both the seller's and buyer's control, not if a customer regrets making the purchase because of a price fluctuation.