When is zakat due on a truck that you bought for the purpose of selling, but you started operating it due to not knowing when it would be sold, keeping in mind that you hope to sell it?

As long as you purchased the truck with the intention of trade, it is considered a trade commodity and Zakat is obligatory on it, even if you rent it out, because it was purchased with the intention of both revenue and trade.

Therefore, the truck must be appraised at the completion of each lunar year from the time you acquired the money with which you bought it. Then, you must pay its Zakat, which is one-quarter of one-tenth (2.5%) of its value each year until you sell it.

If you are unable to sell it and do not have sufficient liquidity to pay the Zakat, there is no harm in delaying the Zakat until it is sold. Then, you must pay the Zakat for all the past years.