Remember, when you bid on an item, in all likelihood your id is shown in the bid history. For all you know, this person may have bid on the item too, but lost. Or they may have just watched the auction. In the meantime, they may have been hoping for something similar to show up again. If it didn't, they decided to ask if you wanted to sell yours. Pretty smart of them, it seems to me.
I wouldn't be offended. It's not like they were stalking you. If you had been thinking of selling, this would have been a great opportunity for you. No need to advertise or pay any selling fees!