Would you count a hybrid conference [0]? It's 100% indie and self-hosted in the sense that I wrote my own ticketmaster for it. Ticket registration flows are more complicated than meets the eye, and it was an interesting problem to solve.
I personally consume payment notifications from Stripe or PayPal, and the rest is processed in-house: updating sales records, generating QR code tickets, sending invites to a private conferencing server, and so on.
This is definitely one of the challenges, since the majority of ticket sales happen in bursts.
Another big one is Stripe or PayPal making mistakes: duplicate events might be sent to my server but that doesn't mean someone bought two tickets. Perhaps an event failed to be sent at all. Or the payment event is fake (rare, but must be managed.)
A final example is that somewhere in the registration flow there's a network glitch: perhaps a confirmation email wasn't sent because the email API was down. How do you patch up these "holes" in a customer's record ASAP? All of these problems might influence and amplify each other too.
Anyway it's definitely solvable but requires programmer effort to scale and fail / recover gracefully.
I personally consume payment notifications from Stripe or PayPal, and the rest is processed in-house: updating sales records, generating QR code tickets, sending invites to a private conferencing server, and so on.
[0] https://handmadecities.com