I think it can also make sense to ask why a product is more expensive to learn of the circumstances that lead to that, as there may be a bottleneck in the supply chain and this guy is literally a material collector.
As for the bad quality, I'm not sure that's what he meant. The other adventurers were mentioning that they were buying low grade potions so it might be that the alchemist is only putting those on display.
But yeah, it's probably just a case of "this fat alchemist is greedy, let's make it extra obvious".