Might be bugged for you (always seems to be a possibility with this game), but I've noticed that - perhaps on certain days of the week? Not sure - on occasion, I've tried to get into his shop in the early afternoon, and it's locked. He seems to keep a rather lazy schedule compared to the other shopkeepers in Whiterun.
This is a bug that's existed since Oblivion. Has to do with the lock timers (or however it works) on the shop doors. Look away from the door (doesn't seem to work if the pick lock prompt is on screen), wait an hour, then check it again. As long as you're between like 10am-7pm, he should open up.
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This happened to me as well, but I figured it was only because I had bartered all his gold away from him. I noticed that any time he didn't have more than ~100 gold to barter with, his shop would just close, then re-open after a few days when all the shop inventories get reset.
Like I said, trolling this isn't. He's taking it too far and it's getting quite annoying. - jubjub360