I have only had one problem with Ebay when a DVD never arrived. The seller gave good communication and refunded promptly when it got to the 'lost' stage. I was content with that. As I never recieved the item I couldn't leave positive feedback but because of the way it was handled i left neutral and explained why.
I have never sold anything on there.
To me some of the responsible points are:
1. The seller should be selling the actual item shown/described. If it is one of several that this should be clearly stated that it is.
2. The seller should make sure the item is properly packaged to arrive at it's destination in good condition (i.e. posters etc should be sent rolled in tubes not folded into an envelope).
3. The seller needs to have proof that the item was sent. You are relying on someone else to deliver it, they can make mistakes etc. Ideally a signature should be required on the other end so at least you know that the item arrived.
4. The seller should NOT have to leave feedback once payment is made. The deal isn't over until the item has arrived correctly.
5. The buyer should leave feedback once the item has arrived in the correct condition. If it doesn't contact the seller first. Then Ebay etc. Allow for the fact that everyone has different schedules or even time-zones. Not everyone can access a computer everyday because real life gets in the way. Having said that if you are selling you should make sure you have the time to answer questions and follow-up after the initial sale.
6. If you are buying always check the feedback. A couple of bad marks can mean nothing, but if they have a bad rep then it's your own fault if you end up getting stung.
7. If you don't have the money ready to pay then don't bid. Excuses like 'I've just got to get the money together' are not acceptable.
Ultimately there will always be scammers and people wanting something for nothing. It's an unfortunate risk of trading.
John