I always leave feedback, especially how "good/new" a used item really was and if the seller went to great lengths to ensure good wrapping and posted the items quickly.
If sellers go above my expectation (which is "average" quality and service), IMO this deserves to be honored. If quality and service are "average", I normally put "item as described".
Also, I normally write what I bought. Please bear in mind that after 90 days, the actual product sold is no longer visible. To a potential buyer looking at a seller's ratings and prior business, it is important information whether the 100+ articles sold by the buyer were 3$ trinkets or, e.g. 500 $ camera lenses. So especially for veluable items I normally also put that. "Perfect Canon EOS 80-200/2.8 lens, fast and secure shipping" fits well into the text box and gives potential future buyers valuable information.
Sometimes, unfortunately, unscrupulous sellers build up a reputation by selling hundreds of cheap trinkets, before then offering expensive electronics/computers etc.
Also, if an account of a legitimate seller who has a long history of selling beer mugs and and football caps gets hacked and suddenly 10 new Macbook Pro computers are on sale at really good prices, chances are that more buyers get tricked by the "positive feedback" than if there are no prior sales or if it is not at all visible from the texts what kind of items were sold.