Footballer's value can fluctuate as frequently as the stock market, with injuries, loss of form and expiring contracts reaping havoc for club's financial plans.
The best dealers in the transfer market know when to sell, and sell high, with Arsenal moving on Alex Song for £15 million last summer an excellent piece of business.
Aston Villa are asking a high price for Christian Benteke, and just like Newcastle stung Liverpool for £35 million for Andy Carroll, selling a player at their maximum market value is an art form.
It gives Tottenham something to consider when mega-figures of £80 million are being discussed with regard to Gareth Bale. Should they sell now, will they regret not doing so?
Here are five players who their clubs no doubt wish they cashed in on last summer:
Martin Skrtel - £25 million
Manchester City wanted to offer Liverpool £25 million for Martin Skrtel last summer, with the Reds dismissing it out of hand. Skrtel went onto endure a nightmare season, dropped by Brendan Rodgers and falling out with the manager. Manchester City signing Matija Nastasic instead and are not thought to be interested in Skrtel, who Liverpool may feel they would be lucky to get £15 million for this summer if they tried to sell.
Nani - £25 million
Manchester United were claimed to have rejected a £25 million offer from Zenit St Petersburg prior to the Russian transfer window closing in early September. United dismissed it because they did not have time to sign a replacement, and the winger had a typically frustrating season. He now has one year left on his contract, recently attracting a bid from Galatasaray for just £7 million.
Thomas Vermaelen - £20 million
Barcelona have made a habit of raiding Arsenal for players, with the aforementioned Alex Song moving last summer. They may well have been full scale protest at the Emirates had Arsenal sold Thomas Vermaelen to Barcelona a year ago, with Robin van Persie also departing. Instead he was made captain and Barcelona were told to back off, but a year later he cannot get into the Arsenal side and many fans would rather see Arteta or Wilshere made skipper. Are Gazidis and co. cursing a missed business opportunity? You bet...
Fernando Llorente - £30 million
Clubs from across Europe were linked with signing Fernando Llorente from Athletic Bilbao, after a stunning 29-goal season which saw him score the goals to dump Manchester United out of the Europa League. Manchester City were strongly linked, but Bilbao's determined to push for big money cost them. In January he signed a Bosman agreement with Bilbao which will be ratified next week, and only contributed five goals the whole 2012/13.
Grant Holt - £6 million
West Ham were keen to sign Grant Holt from Norwich City last summer, having a £6 million bid rejected for the striker. He signed a new contract with the Canaries, scoring eight goals in 2012/13. He is now being linked with a switch to Crystal Palace for just £2 million, with Norwich having brought in Ricky van Wolfswinkel to lead their attack next season.
Who else should be on this list?