i have my main house (grand topal hideaway) listed in the housing tours, it has multiple target dummies, scribing altar, and grandmaster stations (in progress being filled out), as well as almost every antiquity furnishing on display to preview them
I went to see what he says when fencing an item (on UESP) but didn't find any dialogue. However, it could be like DenverRalphy says, that it is then stealing for profit. He also doesn't like you being seen while committing a crime or using a leniency edict, but he DOES like you paying off your bounty to a guard. (from the dialogue I have seen, I don't know whether this triggers rapport changes).
So, it could be that he feels that stealing might be necessary, but if you fence it, it means you are just doing it for the gold (he also doesn't like you stealing medicinal type stuff)
I am disappointed, however, that you don't gain rapport by summoning a kitty, even though he supposedly remarks on it if you do (unless I just missed it)