Back at his flat, he stashed the pouch away under his bed and resolved not to tell his friends until the pick-up man called in a couple of weeks. There would be plenty of time to tell the story when the money was in his hands. He wasn't stupid. But then stupidity was not what Jawal the conman had been counting on when he sent the travelling student on his way from his shop in Jaipur, India.
One thing the young have in abundance is daring, and the brave are just as open to temptation as the stupid. The trick, so to speak, is not to obscure an illegal act and sell it as an unmissable good deal, but rather to convince the victim that their actions will be very much against the law. After all, if someone wanted to rip you off, why would they tell you that there was a risk involved?
To Martin, the element of risk itself was irresistible. Forget Hugo Boss, Ferraris and holidays in Gstaad. The most sought-after status symbol among today's fashion-conscious youth bears the legend "stolen goods". Like designer labels, however, stolen goods are often counterfeit.
Martin had met Jawal in his Jaipur tea shop at the end of his backpacking holiday. Jawal had made him feel more than welcome, offering him food on the house, introducing him to his extended family and, after a few days, letting him in on his sideline in jewel "smuggling".
There was no direct pressure to get involved. In fact, the risks, small though they were, were exaggerated slightly - the smuggler risks having to pay duty on the gems, or having them confiscated. Left in no doubt as to Jawal's sincerity, Martin soon found himself vouching for his own. The other man again tried to discourage him, saying that on an average pouch he would stand to make "only pounds 400" - "probably not worth the risk". With hindsight, Martin says that it would not have mattered if it had been pounds 4,000 or pounds 40 - "I had my adventure-tinted spectacles on, and I was hooked."
Finally, Jawal agreed to send him back with a "consignment" if he left a retainer for good faith in the form of a credit card payment slip signed for pounds 400.
"I have to cover myself, you understand," he said. "I've lost money before and I don't want it to happen again."
The gems would be picked up from Martin's home address by a third party soon after he arrived back in England. The deal having been struck, Martin drank for free for the rest of a long night - "Perhaps Jawal was feeling guilty," he says without much conviction.
The credit slip was probably cashed before the plane had touched home soil, and three weeks later no one had turned up to collect the goods. Cutting his losses in Hatton Garden, Martin was glad to claw back pounds 60. He had, in effect, simply bought a bag of cheap jewels, inferior amethysts and emeralds, at a vastly inflated price.
There are many parallels closer to home. The practice is so familiar in small-time drugs deals, for example, that it can hardly be called a scam. Walk down Camden High Street on the wrong Saturday night and you might believe North London to be in the grip of a drugs deluge. But very few of the drugs on offer are genuine. Though the "dealers" throw furtive glances, they are keeping an eye out not for the police, but for dissatisfied customers.
A scam that crops up every summer has two men driving around in a van bearing the name of a retailer and attempting to sell "top-of-the-range" hi-fi speakers to passers-by. They claim that their company has mistakenly given them one set too many to deliver, and that they are desperate to sell them before they get back to the depot.
"It's totally illegal," they insist, "but there's no way you can get caught."
If they are feeling particularly generous, the knock-down price they quote may be close to the equipment's true retail value. Those who fall for it often expect to sell on the speakers and make a fast profit for themselves. As Jeffrey Bernard observed, nothing quite has the attraction of unearned money, and all the better if it is come by illegally.
Fake prostitutes, or "clippers", work nightly in King's Cross. The prostitute will lead her client into a quiet back street and haggle over price while her companion alerts accomplices. Once money changes hands, then by unfortunate "coincidence" the woman's large "boyfriend" happens to walk around the corner and asks her what on earth she thinks she is doing.
Earlier this year, copies of the notorious Pamela Anderson/Tommy Lee honeymoon video were on offer in pubs all over the land, pitched to believers as the stolen original itself. They sold, and may still be selling, for up to pounds 100 to those who had not looked at the US video best-seller lists lately.
Aside from its efficacy, there are obvious advantages to such ploys from the perpetrator's point of view. For one, a pseudo-crime is far less dangerous than a "real" crime, as the victims place themselves outside the protection of the law. There is, unfortunately, no Trades Descriptions Act for crimes. For Martin, it could have been a great escapade to recount to his friends in the bar room, but his pounds 400 was not entirely wasted. "You know," he says, "for the walk through Customs alone, I think it was worth it."