my wife did the same thing a few years ago, she bought me 2 silver coins from a co worker, because she never knows what to buy me..
First thing I said, instead of thank you was how much did you pay, and where did you buy them .. Long story short, my co worker ripped her off in my mind, and I told my wife please don't buy me coins any more.. so far so good..
She learnt a lesson and so did I.. I haven't spoken to our co worker since.. and my wife hasn't bought me a coin since..
As much as I would have liked the gesture.., the overwhelming thought of her getting taken was enough to put a stop to it..