Any time the economy is down you see massive numbers of sites wanting to buy your gold jewelry. They never mention price or percentage of spot price that they will pay. Many will try to buy low and resell the metal for 4-5 times what they bought it for. This is more than the retailers of new jewelry make selling wedding bands and engagement rings.
To make sure you get the best value you need to know the base facts. One troy ounce of gold or platinum is 31.1 grams or 20 DWT. The spot price is what all metal buying and selling is based on and is priced per ounce for pure metal. This value is easy to find in newspapers and on line. If you have 14K gold you need to know the gram weight and how much of that is actually 24K pure gold. Take the weight and use the gold percentage. 14K=.583 so 20 grams of 14K contains about 11.6 grams of 24K. If 24K is $1000.00 per ounce then it is worth about $373.00 at full price. Of course selling is usually done 10-15% under spot so that is about the real value you should aim for. Platinum is usually 950% so it is 95% pure and is calculated the same way using the weight and spot price.
They metal buyer will pay a small processing fee but still ends up with very close the the amounts I have listed. Figure your true value before seeing the buyer and have your figures written down. I would calculate in both gram and DWT prices since both are used and can be confusing if you are not used to the conversion. Now you have the tools to get a true value for your platinum or gold jewelry.