Originally Posted by HarryT
You may perhaps misunderstand me. I'm not expecting to be able to find out who is using my software illegally. I simply want a practical method of being able to prosecute those whom I do catch (eg selling copies of my software on eBay - that's quite a common thing, sadly).
Frankly, as long as they're selling the original box or the license key (and, obviously, not keeping copies for themselves or selling the same instance multiple times), it is none of your business at all, morally. Just like if you put out a CD or a book or manufactured a car and someone then put it up for sale.
Really, the only ones with any real reason to complain about copyright infringement of the downloading sort are the used music, DVD, video game, and (most relevant to this site) book dealers, as well movie/game rental shops (Blockbuster), and any public library that might have to justify its existance purely based on the number of borrowed items in any given period. THEY are the ones where a downloaded copy is almost certainly a lost sale/rental, not the original producer of the "intellectual property," as their customers generally wouldn't buy new and therefore wouldn't be giving the OP anything anyways.