This time I have two.
Idea one. The shoplift shop. Outfit a shop like a self-serve 7/11, fill the shelves with crap, have a guy sorta idly walk around now and again while spending most of his time on a chair and a coupla people behind a desk in a corner. Desk which remarkably supports no cash register whatsoever. In fact, there isn't a cash register to be had anywhere in the place.
So how people pay ? They don't. To get stuff out of your shoplift shop they have to... steal it. Obviously if the attendants catch on the thief gets humiliated and made to drop his goodies, but otherwise if you make it past the door, that's it.
You could sell tickets to this thing, say 20 bucks a pop (or whatever, 19.95) and run it essentially on the principle of an all-you-can-eat buffet for cheap plastic colored crap and cheap trills. In exchange for the ticket, your customer receives a firm promise he won't be prosecuted for theft, and the right to take home whatever he may manage to carry out on their person.
Contrary to how it may superficialy seem, this is easily +EV for you. PVC is something like five bux a kilo and you can make a lot of volume out of a kilo of PVC. Just make sure your entry fee is large enough to repel would be invader bums (the MPEx model) and you should be turning a slight profit (plus a lot of milfs trying to relive their the best years of their life, back in highschool).
Idea two. This one is quite antisocial (har har) and patently obvious, so it's really a poor reflection on the collective intelligence and assembled creativity of scammers as a group during the past five centuries or so.
What you do is you sell a special pill which increases the effectiveness of any other pill. What do you mean this can't be done ? Of course it can, here's the case of Mary from Worchestershire. What do you mean it'll be illegal in five minutes flat ? OF COURSE it would be, assholes in power don't want people to save on their medical bills - and how expensive medicine is "of late"! What do you mean it's fucking dangerous ? So is getting out of bed in the morning, what of it ? So the doctors will hate it ? Point in motherfucking case!
You may think this is about as fucking stupid as colloidal silver - but then again colloidal silver is still being sold, isn't it.
So there you have it, now go "revolutionize" your environment whydontcha.