Can ANYONE identify this sci fi short story

OK, here we go. I remember reading a sci fi short story in a Marks and Spencer Sci Fi Omnibus. The cover was a very badly reworked poster from the original Battlestar Galactica film of 1978. Now for the story itself.

A small boy enters a toy shop and buys a blaster weapon that has various buttons that, when pressed change the settings of the weapon. I THINK he is about to press one final button when the shop keeper appears and apologies to the buy. He says something along the lines of "I shouldn't have sold you the weapon I thought you were a warrior"

It's driving me mad.

Now, the story may feature either the word WPNZ for weapons or BLSTR for Blaster but I'm vague on that one