The perception that trading in jewelry or diamonds always saves you money is simply not true. Jewelry stores mark up their inventory significantly (sometimes more than 100% of cost). This gives them room to negotiate with potential customers.
When you walk into a jewelry store with a piece to trade they will use this to their advantage. In order to create the appearance that you are receiving a good offer they will inflate their bid while charging you full retail price for the new jewelry. This will make it seem like you are receiving a good price, but in reality it is just a ploy. They are using the extra cash you are paying for the new piece to fluff their offer for your old jewelry.
Another tactic used by jewelry stores is offering store credit. While the store credit sum can appear significant, you will be forced to use it on their overpriced retail jewelry. Stores may even require that all pieces purchased through store credit be sold for the full retail price.
By treating the process as two separate transactions, you can have a clear picture of the value you are being offered for your piece as well as the price you are paying for the new jewelry. If their offer for your old jewelry is satisfactory, you should then inform them that you may be interested in purchasing as well. This is where trading can be beneficial. Because of the extra business, they may be more willing to negotiate on the price of your purchase.