A'gon transactions: When is it legit to quit?

Need some help here. I posted an ad for a speaker and rec'd a response from a A'gon member indicating he definitely wanted to buy. Despite that I indicated I wouldn't ship, he said he'd have a box sent from the mfr and I agreed to ship if he bore the risk of loss and paid for shipping.

After extended back and forth (initial contact was on 3/13), he indicated on 3/22 he sent a check and ordered the box. I emailed him Tues. (4/2) and noted that I had received neither and asking if he might have sent it late or whatever, and sought confirmation we had a deal.

He replied the next day and said the check was shipped out timely, he'd contact the mfr, and said we were still on, but that he was going to stop payment on the check. I indicated that if he did that, how would he arrange for me to be paid? I heard nothing back.

I emailed again today asking what was up, and (stupidly) finally checked his member status. Turns out it was canceled or withdrawn, and there is lots of bad feedback from other A'gon users, including one on 3/17 indicating he reneged on another deal. I followed up with an email that if I didn't get a check and the box by Mon. 4/8, I would consider that he reneged on the deal for failure to tender a timely payment.

My question: Is this a reasonable stance to take in the A'gon community? I don't want to be a hard-a**, but I want to complete a sale and get this speaker out of my study.