We are not talking about buying a packet of cloakroom tickets from WHS and selling them down the local pub here, are we? How much would it cost to run a national or even international lottery to sell the number of tickets being talked about? This option runs the real risk that you could actually LOSE a substantial amount, if you failed to sell enough tickets. And that is without considering the legal and tax implications? Many charities have stopped running their own lotteries for these reasons.
To my mind, your best option would be to go to an auctioneer who specialises in these things, who will have the ear of the buyers and traders interested in them, and hope that would generate a bidding frenzy on the day.
But you also need to remember that they are not worth £15k. They are only worth what some mug is prepared to pay for them on the day you want to sell them.