I've never downloaded a complete software package in such a way, but I have downloaded PDF versions of well known books e.g. MySQL Bible ( from ebay ).

I had a problem downloading at first but the seller resolved the issue. I even lost the copy via a PC crash. But because of the way it was being sold ( using purchase references ) the seller was happy to send me a link and I was redirected and I re-downloaded the document.

My only advise would be to check out the sellers' details and of course ask them as many questions about the purchase as you need to.

Martin