Dude, I'm right there with you. I've had several npc dudes interested in me around Skyrim ever since I put on that damn Mara amulet thing. I thought that merchant would have been all over me lol.
My impression was that he just really likes money.
Since unless youre speechcraft is high and perked out, you will get a lot less for selling items than they are worth, and you buy them at a higher price then their value, the man makes a tidy profit everytime you come to his shop.