Learn German: Sind alle Tickets ausverkauft?

The verb here is formed with sind and the past participle ausverkauft. Since verkaufen normally takes haben in the past tense, what is going on here?

This is the stative passive (Zustandspassiv). If this were the regular conversational past (Perfekt) describing the action of selling, it would use haben (for example, Sie haben die Tickets ausverkauft). By using sein (sind) instead, the sentence describes the resulting state: the selling is over, and the tickets are currently in a sold-out state.

I know past participles usually start with ge- or have -ge- in the middle for separable verbs. Why is there no -ge- in ausverkauft?

Ausverkaufen is a separable verb made of aus + verkaufen. Normally, the -ge- goes between the prefix and the verb (like eingekauft). However, verbs starting with the inseparable prefix ver- never take a -ge-. Because verkaufen becomes verkauft, adding the separable prefix aus- just gives you ausverkauft.

If ausverkauft is acting to describe a state, why doesn't it have a plural ending like ausverkaufte to match alle Tickets?