I think his/her question is what is the difference between "tickets sold out" vs "tickets were sold out".

In your first sentence 'sold (out)' is a VERB. In your second sentence 'sell-out' (usually hyphenated) is a NOUN. That is why it needs the verb 'to be' and an article. The subject of this sentence is also wrong -it's the show which was 'a sell-out', not the ticket. The tickets sold out. The show was a sell-out. You can make a noun from many phrasal verbs in this way. Compare, for example: Our car broke down. We had a breakdown. The prisoners broke out of the jail. There was a breakout at the jail.

But you can say "the car show was a sell-out" which means "all the tickets were sold". You can also say "the car show was sold out". There is no substantial difference between these forms and "the car show sold out" except for a difference in time if you say "the car show sold out just now".

The second sentence is better as: The tickets for the car show (have) sold out. Similar to the first sentence. The car show was a sell out. A sell out means all the tickets were sold.

"Was sell out" is grammatically incorrect. You can never say that.