1. A verb without a direct object cannot be expressed in the passive voice. E.g,
(a) I disappear (active voice, no object).
(b) I disappear you (active voice, "object" is "you", but such a sentence is grammatically wrong because "disappear" cannot have a direct object).
[We can say: I MAKE you disappear, but here the verb is "make"].
2. Since "disappear" cannot have a direct object, it cannot be expressed in the passive voice. So, we can say: Fresh chickens may disappear from Hong Kong (active voice); but we cannot say: Fresh chickens may be disappeared from Hong Kong (passive voice, but wrong grammar).
3. "Sell" can have a direct object, so the verb may be expressed in either the active or the passive voice. E.g. "Ah Soh sold that chicken to Ah Beng" (active voice), or "That chicken was sold by Ah Soh (passive voice). So, we can say: Fresh chickens may be sold in Hong Kong. (passive voice).
6. Note that the passive voice is awkward (and therefore not used) for certain verbs (even though these verbs can have a direct object and therefore can be expressed in the passive voice). E.g.,
(a) Xiao Bao visited his friends (active voice, direct object "friends").
(b) Xiao Bao's friends were visited by him (passive voice, but very awkward).
7. Equally, some sentences are used in the passive voice because the active voice would be awkward. E.g.
(a) Fresh chickens may be sold in Hong Kong (passive voice).
(b) To put the above sentence in the active voice, we have to improvise or invent a subject for the verb: People/Shopkeepkers/etc may sell fresh chickens in Hong Kong.
8. Read up on Transitive verbs (a verb which MUST have a direct object to complete its meaning) and Intransitive verbs.
9. Some verbs are ALWAYS transitive; others ALWAYS intransitive (e.g. "disappear", "feel", etc); yet others can be used sometimes as a transitive verb, other times intransitive.

Hope the above answers your question. :wink: