Transfer windows are hard. Sometimes you can buy a midfielder who was just named thinking he'll lead you to glory for the next five years only to sell him the next season for an $18 million loss. That's what Manchester United did with Angel Di Maria.
Barcelona and Pep Guardiola have thoroughly dominated the last eight years of European football, but even they spent $68 million and threw in their top goalscorer, Samuel Eto'o, to buy Zlatan Ibrahimovic only to sell him on to AC Milan a season later for $44 million.
Chelsea paid an incredible $86 million for Fernando Torres in 2011. 20 Premier League goals in 110 matches later, they sold him for $4 million.
Liverpool got the better end of the deal, right? Well yeah, but they spent $60 million of that on Andy Carroll. He scored six times in the Premier League in 44 matches. They sold him for $25.78 million.
Even when the Reds picked up Luis Suarez for what would prove to be bargain $39 million in 2011 and then sold him for $82 million in 2014, they didn't quite get in right. Using the money to replace Suarez with Adam Lallana, Dejan Lovren, Mario Balotelli, Alberto Moreno and Lazar Markovic.
Quality often beats quantity when you're only allowed 11 players on the field at one time.
Real Madrid have spent $1.38 billion on new players in the last eight years, and only have one league title to show for it.
Madrid's spending over the years has been so bizarre even the great Alfredo Di Stefano couldn't hide his stupefaction when Julien Faubert arrived in 2009.
If their fax machine had worked, it might have been more.
Even Arsene 'never spends any money' Wenger spends a lot of money. $633 million on incoming players since 2011.
Manchester United proudly declared they had brought Paul Pogba back 'home' when they signed him in August. Even if it cost them a world record $155 million. They were happy for him to leave 'home' for free just four years earlier. And no United fans,
All these clubs despite huge networks of scouts, analysts and incredible wealth seem to make mistakes constantly.
And then there is Borussia Dortmund who season after season seem to use the transfer window to it's maximum advantage.
More or less depending on the financial realities clubs have three priorities:
1. Hold on to your best players.
2. If you must sell, sell high.
3. Buy quality players, preferably young (so they will have either long futures at your club or can be sold-on for a large amount of money).
Hold on to your best players:
Dortmund are always going to struggle with this. They don't have the money to compete financially with the big two in Spain, the top eight English clubs or even Bayern Munich in the Bundesliga.
Here are just some of the players that have left in the past few seasons; Robert Lewandowski, Mario Gotze, Shinji Kagawa and Nuri Sahin.
Lewandowski was the most painful as he left on a free transfer in 2014 to main rivals Bayern Munich.
Let's start with the positives; they've kept (despite interest from Real Madrid, Arsenal and Liverpool) both Pierre-Emerick Aubameyang and Marco Reus.
Between them they scored 62 goals and created 20 assists last season.
The fact that clubs were willing to pay well over market value for strikers - see Juventus' $132M purchase of Gonzalo Higuain - this is very impressive.
However it wasn't all good news as Henrikh Mkhitaryan, Mats Hummels and Ilkay Gundogan all left.
And what could Dortmund do? City, United and Bayern were offering a huge increase in wages and all three of those players were on the final year of their contracts. Meaning they would be available to leave on a free transfer in 12 months time.
Which brings up to priority two.
If you must sell, sell high:
Often people confuse the value of a player with what a club ends up paying for them. The price of a player is very much tied to how many years they are contracted to their current club.
For example Pogba was contracted to Juventus until 2019. Meaning had Juventus not want to sell him, it would be three seasons before he would be able to walk away on a free transfer. United had to pay over the odds if they were to get Pogba.
Mesut Ozil was a contrary example in 2010. The star young player at the 2010 World Cup, all of Europe's top clubs were after him by the end of the tournament.
Entering the last year of his contract at Werder Bremen, Real Madrid were able to pick him up for a relative steal at $26 million.
To put that in perspective Mario Balotelli and James Milner both went to Manchester City the same year for $44 million each.
So the fact that Mkhitaryan, Hummels and Gundogan were all entering the final year of their contracts and were sold for a combined $153 million is incredible.
Three incredible players who no doubt Dortmund would have preferred to hang on to, but like any smart business they realised the financial realities and made the most of it.
That allowed them to execute priority three.
Buy quality players, preferably young:
Dortmund took that $153 million and bought eight players. All of them 25 or under.
They brought in Andre Schurrle to give them extra speed (which is frightening) and quality up front. The former Chelsea winger is a proven threat in the Bundesliga, scoring 48 goals and creating 31 assists in 175 matches.
24-year-old Mario Gotze is back to the club that produced him, and while he's struggled for game time his quality is undeniable. Both Gotze and Schurrle are replacements for Mkhitaryan who produced a ridiculous 23 goals and 32 assists last season.
The x-factor, and potentially bargain of the window, might be 19-year-old Ousmane Dembélé who cost $22 million.
The right-winger ripped apart Manchester United, and Marcos Rojo in particular, during a pre-season match.
After he made his Bundesliga debut against Mainz, his opponents couldn't help but heap praise on him.
If this continues, either he'll have an incredible career with Dortmund or they'll be able to at least sell him for an incredible amount.
Sebastian Rode, 25, Raphael Guerreiro, 22, and Marc Bartra, 25, all bring experience despite their young age. Rode, a defensive midfielder who will add stability in the absence of Gundogan, has been with Bayern the last two seasons where he played 52 matches.
Bartra, a centre-back who will replace Hummels, has been at Barcelona where he played 103 matches, winning two Champions Leagues and five La Liga titles.
And Guerreiro was a key part of Portugal's recent triumph at EURO 2016. The left-back possess an incredible free-kick ability as well, making him a threat going forward as well as strengthening the defence.
All of this business, and there was a lot of it, was done well before the season started which meant new players will have a full pre-season to settle into the team. From a selling point of view, it means coach Thomas Tuchel knows exactly which players he'll have at his disposal.
Now all this doesn't mean that Dortmund are automatically shoe-ins for any titles this season, but football at the highest level is about maximising every possible advantage.
Dortmund have thrived in a complex and ever-changing transfer market, while bigger teams with more resources continue to make mistakes. This puts them in an incredible position to challenge the best clubs in Europe.
