Your message is just as polite as can be; it's hard to see how the average Jill would find it offensive. I think your first buyer ("She said she didn't want to do business with such a nasty seller and refused to pay.") was looking to back out anyway. The second ("She paid- but also mentioned she didn't appreciate my message.") is a little strange. Perhaps she didn't read the auction terms, and assumed she had 7 days to pay? Judging from the requests I get after the sale, I've concluded many people don't read before they bid. Or maybe, as one writer said, you just won the lottery twiceyou lucky person!