The "seller leaves feedback first" argument is usually based on the supposition that "once the buyer pays they have completed their end of the transaction".
If you have a policy that you accept returns, or exchanges, then the transaction does not necessarily end with the payment, does it?
If you leave glowing feedback for a buyer after payment, and then the buyer comes back with unreasonable requests or demands after receiving the item, then your glowing feedback is now not a factual representation of the transaction. Your feedback is a now a disservice to other sellers on eBay who rely on feedback to determine the legitimacy of the buyer as much as buyers use it to determine the legitimacy of sellers.
We used to give buyers feedback first until we came across a buyer who defrauded us. We left a follow-up feedback, but the buyer still went on to defraud several other eBay sellers. This buyer is savvy, he chooses to do business with sellers who leave feedback after payment, so as of this point in time he still has no negs.
We have also twice encountered buyers who have made a purchase, paid, and then plead for us to give them feedback right away so they could "build up their feedback quickly". We were were still "nice" back then so we decided because they asked us nicely, to leave them the positive feedback. In both cases the buyers racked up a lot of positive feedback quickly by doing the same with other sellers, then committed fraud and had their accounts shut down.
Our policy is to never leave feedback first. We have no problems sleeping at night with that policy in place and no one has demonstrated a compelling reason why we should go back to the old policy. We have left leaving feedback up to an automated system and not having to think about feedback is quite wonderful, thank you.