Sure, if the chili peppers or the beans are not packaged, you could buy as little as you want. Sometimes you might get funny looks, sometimes they'll give you the bean for free as it's not worth the hassle to sell an individual bean.

But what about prepackaged beans, like 500 gram pack? You can't open the package and expect to pay for part of it. Sometimes the beans are packaged by the grocery store with their branding. That's the same as not letting you buy an individual bean.