Ticket scalping has long been a textbook example for libertarians; let them scalp the damn tickets! I was an easy target, so I've always been convinced, but I didn't give much thought to what opposition to ticket scalping actually implied. I just read a nice illustration of this issue in one of Samuel Brittan earlier books. He uses the example to illustrate the tension between utilitarianism and liberalism and why liberalism should win. I'll try summarise.
Lots of people want to watch Arsenal play each week, way more than their stadium can accommodate. Management could decide to raise prices as a way of choosing who gets the tickets and to maximise their own profit, but that would upset the vast majority of Arsenal fans. So they decide to sell the tickets cheaply to maintain goodwill (lets say they use a lottery to allocate the tickets among only Arsenal fans instead of first come first serve). Now they could -if they wanted to- use some of the proceeds to compensate the small minority who were willing to pay much more to see the game but now can't because they lost in the lottery. In this case everybody is made happy (except Arsenal's owners, but who cares about them right?). Most people are relieved not to have to agonise over how much they really want to see the game, they all have a shot of watching and the fanatics who were willing to pay lots end up with a bunch of money.
But how about letting the people who won the lottery sell their tickets? Most people also oppose that and you could make everybody happy in exactly the same way as above (give the fanatics even more money). But the two situations are not the same. The first time the lottery saves people who want to watch from the discomfort of thinking how much money they would part with and the anxiety of not knowing whether that would be enough to get a ticket. But the second time, they've already had a chance to get the ticket without stressing, so why prevent people who really want to watch from buying from the lucky ones. Now the preference of the majority is that nobody be allowed to express their own unique preferences, it's nothing to do with their own odds of watching the game.
If you think it's legitimate for the majority to prevent the tickets from being resold (and most liberals do), on what grounds would you argue that we should discount the preference of people who would prefer that nobody ever engaged in gay sex?