The term sell out is sometimes called a phrasal verb. Some grammarians prefer to simply call it an idiom. In either case, you can't discern the meaning by looking up the components. Sell plus out taken literally would mean commerce outside, but the term actually has two different meanings.
The first is that the entire stock of some commodity has been sold and none of it is available:
[1a] The tickets sold out ten minutes after the box office opened.
Notice that this is an intransitive usage, i.e., there is no object. That means that there can be no passive-voice construction. With active transitive verbs, you can transpose to the passive by making the object into the subject and making the subject the object of the preposition by:
[2a] ACTIVE: The box office sold the tickets.
[2b] PASSIVE: The tickets were sold by the box office.
You can't do that with sold out. There's no object X to make the sentence
[1b] X was sold out by the tickets ten minutes after the box office opened.
And a good thing, too, since it doesn't make any sense. In the sentence
[3a] The tickets were sold out.
sold out is the past participle acting as a predicate nominative, describing the subject tickets. It has the same syntactic form and meaning as
[3b] The tickets were unavailable.
It is idiomatic to transfer the unavailability of the commodity to the seller. Thus the cashier at the box office may say
[3c] We are sold out of the tickets.
[3d] We have sold out of the tickets.
Both mean We have no more of the tickets left to sell. The grammar remains unchanged.
The second meaning of sell out is to abandon one's principles, talents, or allies. This may be intransitive:
[4a] He was a brilliant writer, but when he went to Hollywood, he sold out
[4b] He sold out his supporters when he voted for the tax increase.
The implication of sell is that the sell-out has been induced to his action by some form of bribery -- by money, position, influence, etc.