As a seller I would never leave feedback first (nor however would I be pushy with the buyer for feedback). I only consider the buyer's part of the transaction complete (i.e. complete enough for me to leave feedback) only once they had left feedback for me. Payment by the buyer is not necessarily the final part of their involvement, especially if disputes arise over the item that need resolving (e.g the goods might not arrive, might be damaged, or might not match the buyer's expectations). As a seller I would feel vulnerable having left good feedback early if I then had a buyer who started causing unjustified troubles. Should your testing of the camera have revealed a fault then your part of the transaction would not yet be complete as you have to manage a resolution with the buyer. That could be sending it back and being refunded or it could be something else. If you then (and this is purely hypothetical) messed about the seller at this late stage then the troubles you caused would not be reflected in the feedback since it was left purely on the basis of prompt payment.
Yes, ebay is a minefield!!