I've already reported this as a bug, but I need some help with this one.
Belethor is the General Store owner in Whiterun.

He was dead before I even met him. When I entered Whiterun for the first time (approximately 1 hour into gaming time) he was lying dead in the middle of the section of the city where the big tree is. It's not even an area where he could have fallen off something and died, so I can't even guess what happened.

I tried resurrecting him with the console, but he never had an option to trade (i.e. no What do you have for sale topic) and now his store remains locked and I think he has disappeared completely.

Bit of a pain given that Whiterun is my main port of call when off-loading loot from dungeoneering.

Has anyone figured out how to respawn NPCs? I know this could be done in Fallout 3 with the traveling caravan merchants, so there must be a way in Skyrim too.

Shouldn't the radiant AI take care of that?

Sounds similar to a stable master I went to speak to, as I engaged in conversation I was about to select an option when he suddenly appeared 50ft above me, the camera automatically panned to him falling and landing with a splat on the floor in front of me.

I stood there for a good 2 minutes thinking about wtf just happened, laughing and panning around. Suddenly some guards accused me of killing him and I had to reload to a previous save as the impact may have been too much. Perhaps Belethor decided to teleport and drop out the sky too, only before you got there =(

You are probably right.
I haven't noticed anyone else turn up dead for no reason in my game yet, thankfully. It's a shame it had to be an NPC that was pretty useful