Yes, they containing stealable items.
First you have to lockpick them, because "Magic Unlock" doenst work here.
After you have opened the box, cast "Telekinese" three times on the box, they are often 3 times trapped.
Then you can look into the box and "steal" items out of it.
Be sure that your skill is good enough, sometimes, the shopowners call the guards on you.
The items inside the box arent really good, just turntips, whips, bottles and such, mostly comparing to the shop you are in.