Took it in the shorts three times in a month?

I have too much of a conscience. I will pay up front first, as I would expect when I sell something. I recently sold a poster that had a tear in the middle which I didn't catch when I advertised it......I caught it when I packed it to mail. I refunded the guy a few dollars because of the tear. He told me later that I didn't have to do it, but it really bothered me that I didn't sell him a good piece that I had advertised.