The pendulum of balance has been swinging wildly back and forth between buyer and seller at ebay. It wasn't too long ago that sellers were routinely screwing over buyers and leaving scathing negative feedback if they tried to get any resolution.
The whole "I'll leave feedback after you do" thing was one of my pet peeves as an eBay buyer. If I'm the buyer and you're the seller, then the entire basis for your evaluation of my performance should be whether I paid promptly. That's it. Once I've paid you for the merchandise, my role is over. (Assuming I don't pull a scam of my own claiming the item wasn't shipped, wasn't as described, etc.).
The seller's role is far more complex, and it's understandable that a buyer may need more time to evaluate - say, until the buyer has had time to receive the item and make sure it is as described.
Back when sellers could leave feedback for buyers, I always though that they should have needed to do so at the time of receiving payment. You won the auction, and you paid me within a reasonable timeframe? A+, we're done here. Assuming, again, no scams by dishonest buyers, etc. - but such matters should always have been handled through eBay and PayPal's dispute resolution mechanisms, not via feedback.
Whenever a seller said "I'll leave feedback after you do", I interpreted that as extortion and moved on to the next listing.