I've just found annoying that so many sellers on ebay refuse to leave you positive feedback until you have done the same for them. Even when you have paid upfront the moment you have won the auction or bought-it-now. It's another way many of these scammers get away with stuff and have such high feedback ratings. People don't want to leave a negative feedback mark when they know they will recieve one in return.
I did manage to get one guy to change his. Was getting G1 Thundercracker and Skywarp and then he went off the grid for months. Other people buying TF's from him were contacting me to find out if I'd had any better luck contacting him. In the end we all put in paypal claims at once before the time-period elapsed and that made him send the items (he claimed he had not done so earlier because his computer had broken down so couldn't use the internet. Had he not heard of internet cafes? Course he had! He was just dodgy!).
I ended up leaving him a neutral mark, saying it was a good product but very poor communication and postage time. He left a nasty one about me. I contacted him and pointed out I could easily have left negative feedback and I was willing to take this further so he relented and changed his to a positive.