Belethor just f*ing disappeared. I've waited weeks in the game and he hasn't come back. Will that merchant EVER respawn?

He had an unfortunate accident while I was testing out his, guaranteed to cut the head off your enemy, or your money back sale.

He either got hit in the Knee with an arrow or... sold himself to slavers or something but the merchant in Solitude in "Bits & Pieces" will buy everything he bought and more

He might have locked himself in his house/room, i've seen npcs do that plenty of times.

His assistant was tired of saying "I work for Belethor at the general goods store"

Since all he does, is chop...wood...all day.