Homeopathic H1N1 cures/immunizations.
Guaranteed no side effects, the option big pharma threw out because of the low profit margin.
Phony amber alert bracelet tracking devices.
The poison part isn’t true. Chinese companies were selling milk laced with melamine not long ago. Toothpaste was also recalled.
Form a cult. I’ve thought half-jokingly about it, now and again. My father too, for that matter.
Hey, L. Ron Hubbard could do it, and I figure I’m at least as good a hack sci-fi writer as he was. And while I was growing fat and rich off the devoted toil of my cultists, at least I wouldn’t be actively tormenting them or others.
As Dad likes to say on the subject, “If God hadn’t wanted them shorn, he wouldn’t have made them sheep.”
First fifty followers who join up get to pick their own colored robes! Now return to the Lima Bean harvest. Praise leader.
I’d be like Shackley, selling high-end vitamins that are no better than off brand vitamins.
I just think it seems so…easy.
I’d start a religion. Best scam going.
I remember once Oprah was doing a show on scams and a guy dressed up as a security guard and sat by a bank night deposit drop box and told people “oh the box is broken, I’ll take that” and just wrote people’s names on a clip board and they GAVE HIM THEIR BANK BAGS. Oprah was flabbergasted. “How can a hole in a wall be BROKEN!?!?” So yeah. If I had the balls I’d do that.
The spanish prisoner. Not the original scam, but the David Mamet version, just because it would allow me to say things like, “We must never forget that we are human, and as humans we dream, and when we dream we dream of money.”
Stranger
I can think of a few ways to increase the time-to-realisation, and thereby reducing the potential for identification, but I’d rather go for the easy money. The advantage of selling placebos is that the only people who will get close enough to mess up your scam are the ones stupid enough to fall for it.
I wonder if that was before or after Neil Gaiman wrote American Gods?
Oh, and I can’t tell you what I would do, because … it’s completely possible and no-one’s done it yet (to my knowledge). I don’t want to give you kids ideas. 
The first rule of a scam is to put or find someone out of their comfort zone. Then you offer them your confidence (“I really shouldn’t tell you about this opportunity, but <the lure> says you’re a straight shooter, so…”), and wait for them to reciprocate.
Stranger
My Grandpa’s Name was HaRold! Wow! That’s amazing!
Could you tell him that I’m doing just fine? If you charge for doing that, I can post my credit card number…
Televangelist, definitely.
Medicare fraud. You need zero skills and the returns are huge. Rent a storefront, purchase names and Social Security numbers from the crooks who peddle them, start billing for all manner of outrageous things. Move your storefront every month or so to stay ahead of the investigators.
Funeral director.
Ugh. Good call.
It was a long time ago…like at least 7 years, maybe 10. The guy had a bunch more elaborate scams but they were a lot more work. Like small-time scamming shopkeeper type deals you wouldn’t even bother with.
“Scam” has such negative connotations, I prefer to use the term “Low IQ Taxation”.
I’d do anything where all that was involved was me asking suckers for cheques for certain products, and then not sending them anything.
Couple of rubber bands, plastic handles from the hardware store, some nuts and bolts maybe a pipe, and, voila, amazing new weight loss device. Make up a couple of endorsements from pseudo Dr’s.
Get a couple of smiling, skinny, half dressed chicks for the video. Not available in stores, been used for eons in Europe, yada, yada, yada.
Cannot miss.
You’ll be rich in no time!
Get some official looking letterhead and start mailing out invoices for mundane stuff like “office supplies.” Some of them will just get paid because A/P aren’t paying attention and, hey, we bought some office supplies, right?