I usually leave feedback as soon as the buyer does for me, or I expect it as such. I agree that the buyer has done his part with payment. I don't hold feedback hostage, that's not how I operate.
It might be semantics in terms of timing, but I guess I just conceive of feedback as a post-transaction process.
Autograph Dynasty, LLC (SF Bay Area Autograph Signings/Memorabilia) - http://www.autographdynasty.com/