Someone may ask you for more than 500 diamonds for an item (ex. 1000 for a black long). They put a necklace in their store for 500 diamonds and say they'll add the other item after, because you might take their item and run. After you buy it, they lock you out of their den.
They may also break it up. For example, they'll put a bad item in their for 250 diamonds, then promise that they'll sell their big item next, and do another bad item for 250 diamonds after. However, they may end up locking their den after you buy their item for 250 diamonds.
If you sell an item for a price exceeding 500 diamonds, someone may scam you by paying for your large item first, but never buying your bad items.
How to avoid this scam
The only way to assure that you won't get scammed is to not do big buys and sales. However, there is usually a safe method to sell large items. For example, if you want to sell your Black Long for 1500 diamonds, sell 3 Red Longs for 500 diamonds each. Then have the buyer trade those 3 Red Longs for your Black Long.