Don't hand over your used (but still valid) day travel cards to the touts who hang around some London stations in the evenings.
Giving them your card is actually illegal (they are 'non-transferable'), and the touts re-sell them on the black market.
Buying a travel card from these dubious characters is also illegal, and the card is anyway unlikely to work.
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Moreover a report by the the Anti-Counterfeiting Group may give you pause for thought: fake perfumes are commonly laced with urine, which acts as a substitute for the stabilizers present in genuine perfumes. Mock Auctions are a variation on the 'illegal street trader' theme.