Merchants are an ugly breed.
Buying low and selling high means ripping off two people for personal gain, which generates a negative impact on the game, the community, and even the developers, just so that you can look nice, or make "profit" so as to be the richest knight. I don't think this is how we should be playing the game.
Rather than waste your time begging people to pay too much or sell for too little, you could get a job and spend money on the game, propagating it to some extent. If you want something, you can and should work for it and buy it.