I never used to but I realised how much I rely on other people's honest feedback before purchasing items so it seemed wrong not to leave my own comments to help others! So now I try to leave a review of the item and feedback on the seller. Maybe not every time but definitely in the majority of cases! And not just bad reviews either, if something is just exactly as described or even better then I say that too.
Yes, always. I think it is a courtesy when you are pleased with something you have purchased. Conversely, I think it is always appropriate to let the seller know when you are unhappy with a purchase. So far, I have been pretty lucky with anything I have purchased on line.