You can try to sell them through eBay. I hate to tell you, but they are worth very little. You can purchase them from a dealer or on eBay for about $2 per cover or even less. They have almost no real gold, such a thin layer that it would take thousands of them to get a measurable amount.
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That is going to be hard. They are not easy to sell as they are not worth much. It takes thousands of them to make any noticeable weight of gold, so the value is entirely in what collectors will pay for them. Most stamp collectors have no desire to have such things. You can purchase covers for a dollar or two each on eBay, so you might try there.