I just learned that, although extremely amusing conceptually, you can't actually sell goblin thieves to the merchant caravans. (Or, at the very least, that I'm doing it very, very wrong.)
Had these goblins sitting around in my stockpile for a while, and was contemplating various amusing ways to dispose of them. Caravan shows up, and I'm thinking 'hey, problem solved!'
No, not really. I see a message saying "Urist McHauler cancels Bring Item to Depot: Interrupted by Goblin Thief"... and before I even have time to think 'damnit', there's a whole flood of similar messages. Sure enough, my haulers went and freed the goblins before attempting to move the cages.
Worse still, the goblins are apparently armed and stabbing people. Fortunately, the goblins were freed in the animal stockpile, which is full of stray war dogs milling around, plus one war lion (yay, elves!). Otherwise, that could've been kinda Fun. Actual kill credit goes to a legendary miner who was passing through, though... everyone else is freaking out and running around, and the war beasts are failing to hit the goblin through his impenetrable silk armor, while this guy walks up and one-shots with 'The Miner strikes The Goblin Thief in the head with his (copper pick) tearing apart the muscle and shattering the skull through the (cave spider silk hood)!'
One almost got away, even half-mangled and bleeding, but did not manage to escape through the field of cage traps around the entrance. I expect him to die shortly.
In any event, I need to rethink my goblin disposal strategy. That's a bit too much fun for everyday goblin disposal.