If you've ever been burned on eBay by a seller with less than stellar feedback, chances are that the first thing you do before bidding is check for negative feedback. EBay doesn't make it easy to isolate those comments, but Toolhaus.org lets you enter an eBay user name and lists only the negative feedback, if any, for that user. The eBay Negs script makes it even easier by placing a clickable icon next to every user ID that takes you directly to the Toolhaug.org page showing the user's negative feedback.
Another eBay annoyance is that shipping charges can sometimes drastically change the final cost of an item, and it is not always easy to tell at a glance what that final cost will be. The Display totals with shipping script is handy because it shows an extra column on a search results page with the total amount you will pay, including shipping. Keep in mind that this total is only completely accurate on "Buy It Now" sales, because it takes the current price and not necessarily the final selling price on auctions. Also, if the item doesn't list a set shipping fee, the total will not show in the extra column.
My husband loves to watch movies, and he always looks them up on IMDB. He likes The Movie Dude script because it puts a bunch of links to other movie-related sites' information on whatever movie he happens to be looking at. This script links to Netflix, Yahoo!, Amazon, Blockbuster, Rotten Tomatoes, Pirate Bay, Ebert, Wikipedia, All Movie, GreenCine, Metacritic, Newzbin, FilmAffinity, and Walmart, so you can get just about all the information that's out there for any given movie.
The Hide credit card numbers script is simple but effective. It just "stars out" your credit card number, like many dialog boxes do with passwords, so that other people cannot look over your shoulder and read the number as you are entering it. Of course the script doesn't do anything to keep your number secure once you've entered it and passed it on to the server.