In the first case: subject (Juan) conveys the action (duchar), and the result of such action falls to Juan himself (hence the 'se').
In the second one: an action is performed (vender) but there is no agent mentioned, and following the verb, there is a passive subject (two objects in which the action is placed, oro y plata).
First sentence is in active voice with a verb used in a 'reflexive form' (forma refleja o pronominal), because action falls on same subject (a defined one) performing it. Verbs in these cases are preceded with a personal pronoun: me (if subject is first-person singular), te (2nd-person singular), se (3rd-person, both singular and plural), nos (1st-person plural), os (2nd-person plural)
duchar (to shower)
Pedro ducha a su perro (Pedro showers his dog)
Yo me ducho (I shower myself)
Me ducho (same as above, me indicates that I perform the action, thus, Yo may be omitted)
Juan se ducha (Juan showers himself)
Se ducha (Someone known showers himself/herself/itself). It is not a good idea to omit the subject if he/she/it is known, unless he/she/it is very obvious in the conversation (i.e. as a short answer to the question ¿Qué hace Juan? - What does Juan do?
Ellos se duchan (They shower themselves)
Se duchan (A group of known entities shower themselves). Same case as 'se ducha', but duchan indicates a plural.
In the second sentence, to say in Spanish 'Gold and silver are sold' (a passive voice sentence), you could say it like:
'Oro y plata son vendidos' (voz pasiva analítica o perifrástica) [not used, however gramatically correct]
But if the subject is a 'thing' (or 'things', like your case), or an unknown/undefined entity/entities, you will prefer:
Se venden oro y plata (voz pasiva sintética o refleja)
Another examples of voz pasiva refleja:
Se buscan actores para la película (Actors are wanted for the movie)
Se necesita carpintero ([A] carpenter is needed)
Furthermore, in Spanish, there are impersonal sentences (oraciones impersonales), which I think they are worth to mention briefly here, whose construction is very similar to pasiva refleja sentences, except their verb is always expressed in 3rd-person singular, even if the subject is in plural:
Se entrevistó a los candidatos para el puesto (Candidates were interviewed for the position)
In all these cases, the word 'se' is merely an auxiliary word, it only shows passivity.
In short, if you can tell in a sentence who (or what) is performing the action, and that action falls on himself/herself/itself/themselves, 'se' is depicting a reflexive form, and the sentence is in active voice.
If you cannot tell who is performing the action (Who is selling gold and silver? Who interviewed the candidates? Who wants actors?), then 'se' is an auxiliary word indicating that your sentence is in passive voice.
I hope it helps.