After you buy the last item for Solitude house, he still says “ask me again if you want to buy more.â€
When you ask him again, he still says “what would you likeâ€, and then the empty list comes up with only the option for “nothing right now, thanksâ€.
I recall in Whiterun, the guy said “sorry, no more†once you bought everything. I wonder if this exists for Firebeard, (text but more importantly VO), such that the problem may just be a logic issue rather than the outcome not existing at all.