One option is both leave feedback anytime, but feedback remains invisible until both have left it, then both feedbacks become visible. That way one cannot influence the other, and if either does not leave it, they get non in return either.

many eBayers who both buy and sell will tell you they leave more feedback than they get.  Feedback is voluntary, so a system where feedback is only displayed IF both parties leave feedback would be bad on so many levels.

Think about it, a very dodgy seller DOES NOT send items and gives their buyers the runaround past the 45 days for a PayPal claim - we've read about that many times on the boards here.  So the buyers leave negative feedbacks . . . and . . . .surprise surprise . . . . the seller leaves NO FEEDBACK for the buyer.  Under your proposal no feedback would be displayed AND the sellers gets no negative.  Great system, eh!