Ticket Resale

Third degree price discrimination can make consumers on average better off or worse off.

True in an aggregate way, but distributional implications so not necessarily true for even majority of people.

You can find equilibria where allowing resale makes some worse off. Of course it is interesting why the venue doesn't charge the mkt price.

Without controls, scalpers may buy up all tickets and sell them at high prices. High income people gain, but low income people lose.

Sounds like there are possible gains from trade. Although I'm no expert, I don't see the negative externality caused by opening this market.

Free trade leads to more efficient outcomes in general, but gains from allowing resale would be distributed unequally

It all depends on how the tickets were initially sold. The question is why there was excess demand in the first place.

Of course, the big mystery is the strong tendency to underprice tickets at the box office. Correcting that tendency would be socially good.

Classic example in which good (tickets) are not allocated to those who value them most. Only exception:ticket-holder's identity matters.

Yes, though the gains in allocative efficiency could be mitigated by transaction and queuing costs.
-see background information here

Since no-resale rules may affect price structures, this may not be as simple as it seems.