what do you do with long bones on the game?

They're used in Construction.

They can be sold to Barlak a cave goblin within Dorgesh-Kaan for 1,000 coins and 1,500 Construction experience.

You need level 30 construction before you can trade them with Barlak.

where is barlak??

dorgeshkaan.. its somewhere mid-left.

If you haven't done it yet, here is his exact position:

 

 

 

 

 

 

 

 

This comes from Tip.it's special map of Dorgesh-kaan with me editing it to show the route to him. Just follow the red line and you'll end up at Barlak (which is the room with the letter E.