Strangers online
해요체 · 합쇼체
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.
Common mistakeReplying in banmal because the other person did. Keep haeyo; matching rudeness usually starts a comment fight.
Examples
좋은 정보 감사합니다.
joeun jeongbo gamsahamnida.
Thanks for the useful info.
혹시 어디서 사셨어요?
hoksi eodiseo sasyeosseoyo?
Mind if I ask where you bought it?
저도 같은 생각이에요.
jeodo gateun saenggagieyo.
I think the same.