Translation:Who was arrested by the police yesterday?
The sentence is "Anh ấy đã bị [ai đó] nhìn thấy trong khi đang bán cái điện thoại của tôi.", which can be translated as: "He was seen [by someone] while selling my phone."
I can write a formula here:
English passive: O + be + past participle [+ by S]
Vietnamese passive: O + bị/được [+ S] + V
Hmm, the whole "Passive" module not only accepts, but has as the first (preferred?) answer, sentences like this:
"Chúng tôi đã được liên hệ bởi một người đàn ông nổi tiếng."
Translation:We were contacted by a famous man.
"Quyển sách này được đọc bởi tôi." Translation:This book is read by me.
You can re-arrange both to fit the O + được/bị + S + V format, and it takes it, or you can use bởi, and it takes it, in all the exercises in that module (I've done just that). As there were no course notes, there was no reason to believe one form was preferred over the other.
it was preferred because it is closer to English and probably easier for non natives to translate. nonetheless, it is more usual to use the construction O + được/bị + S + V. that being said, O + được/bị + V + bởi + S is rather used when the speaker insists more on the doer than the action itself.