Most media stories do not get written unless a journalist gets multiple attestation for a story. For those who don't know, multiple attestation is confirmation of something from more than one credible source. So, for example, in the case of Sancho a decent hack is not going to write that terms have been agreed if they haven't heard this from two good sources. For instance, someone in the player's camp and someone working on the deal etc.
That does not always mean the stories are true though. Sometimes it suits a club and an agent, both of whom have to be taken as credible sources because you'd think a player's club and their agent will know what's going on with them, to claim something is happening when it is not or the opposite. In that position a journalist could think to himself 'this is pure BS I'm not writing it.' However, what if it turns out to be true? Then the journalist has turned down a story, confirmed by the selling club and player's agent, which other journalists will also pick up. How does said hack explain that to his or her editor? 'Yes I had the story but I didn't break it because I didn't believe the sources, even though they were credible.' Once you get to a certain level within the media you might be able to get away with that. However, those are the exception not the rule.
Every report we read will be sourced but it will only be as good as the source and sources have agendas. When Gaitan's camp and his club were telling the whole world he was coming to Man Utd they had an agenda. It wasn't true but it was what they were telling the media and if you're a journalist you are going to report it, just in case.
The people giving reports on Sancho are not all WUMs. They are not writing up stories claiming terms are agreed or this is the price Dortmund want solely for promo. They are writing this stuff because people involved in the situation or close to it are telling them these things. Just because these things don't end up happening it doesn't mean the journalist in question wrote them up for the purpose of misleading people (even if their sources wished to mislead). Also a lot of the time these things are not etched in stone. The intermediaries working on a deal might think they've got an agreement, then they take this back to the clubs and find circumstances have changed or the clubs aren't willing to compromise as much as they originally thought/understood. So to the player, the agents, the negotiators it might seem done, they tell the media, and then suddenly its not done.
