How do you write would like to buy a in Korean polite informal way?

There isn't a single perfect translation for "would like to buy a" in Korean that's both polite and informal, as the level of politeness depends heavily on context and your relationship with the speaker. However, here are a few options with slightly different nuances:

* 사고 싶어요 (sago sipeoyo): This is a polite but relatively casual way to say "I want to buy." It's suitable for friends, family, or people you're close to. You'd add the item after this phrase. For example: "사과 사고 싶어요 (sagwa sago sipeoyo)" - "I want to buy an apple."

* 사고 싶은데요 (sago sipeundeoyo): This adds a softer, more hesitant feel. It translates more to "I'd like to buy, but..." or "I was hoping to buy..." It's still polite but more indirect.

* 살까 해요 (salkka haeyo): This means "I'm thinking of buying" or "I plan to buy." This is slightly more polite than "사고 싶어요" but still informal.

* (~~) 하나 사고 싶어요 ((~~) hana sago sipeoyo): This adds "hana" (하나) meaning "one" to specify you want to buy one of something. For example: "사과 하나 사고 싶어요 (sagwa hana sago sipeoyo)" - "I'd like to buy one apple."

Which one to use?

For most informal situations with friends or family, `사고 싶어요 (sago sipeoyo)` or `(~~) 하나 사고 싶어요 ((~~) hana sago sipeoyo)` are perfectly fine. If you want to be slightly more polite or hesitant, use `사고 싶은데요 (sago sipeundeoyo)`. `살까 해요 (salkka haeyo)` is a good option if you want to express a plan or intention rather than a desire.

Remember to always add the item you want to buy after these phrases!