Yes I have done this as well. The seller was so impressed that she offered to always give me deals (although I have not taken her up on that offer). In fact, she had to relist an item I had made a Best Offer on due to a NPB and said she would accept my previous best offer since I was so nice! Unfortuneately for me, someone had chosen the BIN option before I got the message
Although I do agree it was a totally warm-hearted gesture, I still feel that she shouldn't have done her research on the item by starting an actual listing.
I think it's wonderful, if everyone on eBay was like you it would function flawlessly for sure!
I once sold a Dooney bag, and a kind soul let me know that I spelled "Bourke" wrong (I spelled it "Burke"), I have never forgotten it, and would do the same for others given the chance.