Merchandise stolen by someone should be debited to:

Merchandise stolen by someone should be debited to Loss by theft account

 

Since theft of goods is a loss to the organisation , so loss by theft a/c is debited. Also, stock purchased for sales were debited in Purchases a/c at the time of purchases, now since this stock not available for sale, so expense of purchases has to be reduced, that is why Purchases a/c is credited.