Here is you first post try reading what you wrote and you call me when you call me slow.
"You don't think that there are thousands and thousands of used items for sale all over craigslist sites and on eBay for which the seller has no purchase receipt and for which no buyer will demand one? "
Then you changed it around not once but twice. Sorry you have no since or morals.
Again, I'm lost. Is it the "for which" that's confusing you? Sorry man, but there's no other way to read that sentence than what I intended. It plainly does not refer to all sales. It refers to some of them.