this guy owes people hundreds of dollars from months ago and he is still selling stuff??? there is something wrong here. people who are repeat offenders or have deals months old that are still unsettled should not be aloud to use the buy/sell forums in my opinion.
if you cant ship the item right away let the buyer know. if weeks have passed and you have gotten no response about why it couldnt be shipped or no response period, then i think that you have reason to leave negative feedback. if the guy responds after, say, 3 weeks and has a damn good reason, then leave another feedback explaining. there are no rules as to how many feedbacks can be left about the same deal are there?