Some questions in newspaper advice columns are easy enough to answer. "No, ee's not worf it" being a useful one that applies to many. This is equally simple:
How can Viagogo get away with charging such big fees?
Because they can - some people will pay them. Enough people will pay them to make the tactic viable that is.
At which point a slightly deeper dive into the explanation. People will charge absolutely as much as they can for anything. We can ascribe this to capitalism if we desire - that lust for profit - but it's more a feature of being human. We'd all like to get more coming to us for what we've got to send the other way. So, therefore, people do charge as much as they possibly can.
The solution to this is market competition. It is true that there's some limited number of anything. Most especially tickets to an event. Price is not the only but it's the most efficient manner of sorting through who really, really, wants a ticket and who would prefer to be doing something else given what they'll have to give up to gain one.
OK. But how is that margin, mark up, that the intermediary able to charge limited? By the fact that they don't have a monopoly. Sure, they've an effective monopoly over that specific pair of tickers but not over all those to that event. Thus the competition of others also willing to sell their tickets brings down the margin that can be charged for any specific pair.
Or as we've been known to put it, it's market competition that reduces the gouging that human nature makes us all prey to. Which is why we have market competition of course.