Fans of daytime TV will be familiar with those cash-for-gold ads. If you have old gold hanging round the house that you haven't pawned yet, you just ring them up and they send you a padded envelope. You pop in your unwanted gold, send it off, and by return they send you a cheque for a quarter of what it's worth.
For this all-solid-gold bike, for example, spotted on High Holborn yesterday, you'd get about fifty quid. If you could find a jiffy bag large enough.