The market value of any player is the maximum cost that any club values that player at for the intended period of engagement.
So if Messi is on a free but his market value is say £300M for a 3 year contract then he’ll be employed on that basis.
If there was an upfront fee, payable to Barca, of say £100M attached then, provided Messi was willing to take only £200M for those 3 years, his total cost would be £300M (his market value) and that’s what he’d go for.
I think maybe you’ve confused asking price with market value.
Asking price is what the selling club would be willing to take for a player whereas market value is what a buying club is willing to pay.
Thus, some asking prices mean that some players are actually not for sale.
A players total cost is the combination of all costs - up front fees, players salary over the period of the contract, agents fees, tea & coffee, etc.
Provided the total of all costs meets a players market value then the player will be sold.
If the combined costs exceed market value then the player is, in effect, not for sale.
Oh, and years left in a players contract do not effect a players market value. What it does effect is the contract holding clubs ability to set a price that makes the player not for sale.
That control diminishes as the contract runs down until there is no control at all at the terminal date of the contract.
Think about it this way.
In simple terms.
I’m selling a skateboard. It is definitely for sale because I’m selling it for £250 and people are will to pay £250 for it - that is what the market values it at.
I make a profit (my take)
It costs me £100 to buy the board + a £50 import fee so I make £100 on the deal.
Imagine that skateboard is a player and I’m his agent.
My import fee suddenly reduces to £25. Do I reduce the £250 price to £225 when the price the market is willing to pay (the market value) is £250?
Not on your effin’ nelly I don’t - I’m not mad, I’m in the business of making money, just like all the footballers and all the agents and all the clubs are.
My skateboard will still cost £250 because that’s what the market is willing to pay. The import fees are reduced by £25 and my take from the £250 goes up by £25 so I now make £125 rather than £100.
Only a total nob would sell a skateboard at less that the market would pay for it - reducing one part of the cost does not change the market value it just alters the split between me and those collecting the import duty.
The buyer pays the same market value because me making more money does not effect market value.