I think the problem here is everyone has different definitions of "timely manner." You considering 20-30 minutes a timely manner to get a response after I've already told someone the item is theirs is batshit insane to me.
You misunderstand, I'm saying if I responded to the person who pm'd me inquiring about an item within 20-30 mins, then I'd assume he'd have gotten the message and if after a few hours (in my case I'd probably wait 4ish) I didn't get a response, then I'd go to someone else.
As new information has come to light though (the seller himself took an hour and a half to respond, and then only waited 2 hours), I'd say he was in the wrong. But to address the original point, no he shouldn't receive negative feedback, because that's not how the feedback system was intended to be used.