Strangers online

Also common: Formal (hapsyo-che) (-습니다 / -ㅂ니다)

Why

Comments, reviews and questions to strangers are usually in haeyo, since you can't know anyone's age. Thank-yous and anything to a company's account are often in hapsyo. Banmal to a stranger online reads as rude.

Say it like this

• 좋은 정보 감사합니다.joeun jeongbo gamsahamnida. · Formal (hapsyo-che)Thanks for the useful info.
• 혹시 어디서 사셨어요?hoksi eodiseo sasyeosseoyo? · Polite (haeyo-che)Mind if I ask where you bought it?
• 저도 같은 생각이에요.jeodo gateun saenggagieyo. · Polite (haeyo-che)I think the same.

Common mistake

Replying in banmal because the other person did. Keep haeyo; matching rudeness usually starts a comment fight.