I have a question about how to interpret the word da in a context where it seems like it could mean either by or from. Consider this example sentence:
Da chi sono stati comprati questi guanti?
It seems to me like both of the following translations are possible:
By whom were these gloves bought? (i.e., who is the buyer?)
From whom were these gloves bought? (i.e., who is the seller?)
Which interpretation is correct?
There is no unique correct interpretation: it could mean both and only the context can suggest the intended meaning.
The preposition da is used with personal nouns or pronouns to mean motion to or state in: vado dal salumiere or sono dal dentista. By extension, we can use it also for ho comprato i guanti da Xyz. The English translation for such da would be at Xyz’s.
You're also right in thinking that da is the main proposition for denoting the agent in a passive form.
Thus the example is indeed ambiguous, because there is no grammatical way to distinguish between the two meanings. It's a bit artificial, though.
In the by meaning, an active form Chi ha comprato questi guanti? would be preferred. In the at/from meaning I'd personally ask Dove sono stati comprati questi guanti?
