Perhaps half of the potions should be bullcrap or ineffective that some bad alchemists are selling, but they're placebos to people who don't know that. A doctor might say you can cure your stomach problems with medicinal beer, or cure your headaches with a lead based solution. Or they can require ingredients so rare that there's no reasonable way of attaining them, so desperate people believe the lie if he says he has some. Who's up for having some fun and danger with the business of alchemy in the next game?