Original seller is free to sell it with the stipulation that original buyer is the only person that can use the ticket.
However, entertainers have an image problem that conflicts with the desire to maximize profit. Entertainers are often times in the business of appealing to the broader public. But if they restrict their ticket sales to the highest bidder, then the broader public's appeal is limited.
Having a third party engage in the price discrimination allows the entertainer's image to remain intact, while also collecting higher prices (perhaps not from the ticket sales directly, but the third party willing to pay more for the entertainer if the entertainer allows them to resell tickets).
However, entertainers have an image problem that conflicts with the desire to maximize profit. Entertainers are often times in the business of appealing to the broader public. But if they restrict their ticket sales to the highest bidder, then the broader public's appeal is limited.
Having a third party engage in the price discrimination allows the entertainer's image to remain intact, while also collecting higher prices (perhaps not from the ticket sales directly, but the third party willing to pay more for the entertainer if the entertainer allows them to resell tickets).