Re: shopping second-hand (pawnshops, coin shops, antique shops)
I also find it shocking when things are incorrectly represented such as the pendant you mentioned....But sometimes their mistakes can work in your favor, such as my recent find - a nice 18K chain to use for wearing my pendants. The store's tag described it as 14K. The 18K stamp was clearly visible with my loupe and the gold was yellower/brighter as typically 18K looks in comparison to 14K. I think that frequently these stores don't "do their homework" correctly (a lot of things come in and they rush rather than spend time on each piece) In your case, it might have been deliberate misrepresentation, of course.
I used to be really honest and would point out to the merchant ("Your chain is really 18K, not 14K") But I stopped after having enough of them raise the price on the item after I pointed the mistake out.
So now, I keep my mouth shut and buy.
If the item is lesser (tag says 14K but the stamp clearly says 10K) I gently point it out to the clerk (To spare the next customer from buying something that has mistaken gold content) When this happens, the second-hand merchant always has gratefully thanked me.