Mark J. Searce sterling silver mint julep cups are marked with an eagle cartouche and the initials of the president in office at the time of production. They are sent to the sitting president. Once a president is out of office, his initials are retired and replaced by those of the newly elected president. They were first sold on the retail market during Truman's presidency. JFK cups are highly sought after.
These cups can command hundreds of dollars. They are often sold in a group at auctions. They sell at online auction and sales sites. But most online auction sellers have an account and an inventory, not a single item. You may want to contact a high-end silver dealer. Remember that you will realize only a part of what the seller gets. The greatest amount you can earn from a sale would be to a private collector.