Leave feedback only when you are fully satisfied with the item you have bought. You should not leave feedback when:-
1) you have paid for the item but not yet received it. (If you do that, you may never receive your item and then you can't go back and leave negative feedback).
2) You've received your item but not checked out the item. It could be broken, not working, if it is a cd it may be too scratched to play properly and might jump about. If it's a video the tape could be mangled in places. There maybe items missing, you may even have somebody elses parcel, as quite often ebayers send out huge amounts at a time it is quite common to get orders mixed up.
3) If you have received clothes, shoes, check that they fit, first because if they don't some of the more helpful ebayers will let you return the items, and refund your money.
4) Don't whatever you do like to tidy things up on your ebay site and go around leaving feedback for people in advance.
These points may sound incredibly silly and like common sense. But I have come across so many people who have done such things. We all make mistakes, but no one wants big losses and disappointments do they?