I am selling billiard sticks, they are made from wood. But most of them is a bit bent, is there a way to make them straight ? And how to keep straight ones so they will not bent?
The simplest and most hacky one: Hang them off a string, so gravity can pull them straight over time (one to a couple of weeks depending on wood and bend). This should also be the best way to prevent your queues from bending while you store them. (I have not tried this myself though)
The article linked to above, also proposes bending them back by hand.
From own experience I know that wood will bend easier when you heat it up.
Having said that, wood is a grown material where you have rarely a consistent quality of wood in a piece. This means on change of humidity and temperature the queue could just bend and straighten. This is especially true for mass manufactured wares. An experienced queue turner probably selects pieces based on his experience where he would expect little to no bend; these would be a lot more expensive than their mass manufactured counter parts.
The bottom line is, some queues just will be more difficult than others.
If your queues are from mass manufacturing, you may want to do what the big boys do these days and just select the better queues out and charge more money for them.
There are different kinds of wood, like marple and ash, different ways to prepare the wood and even deactivate it. Knowing about these things will definitely help you, but this discussion would be out of scope for Lifehacks.