The player's market works in a quite similar way as in the real life, but it is always open, twelve months a year and 24 hours a day, so you can buy or sell players in any given moment you want or need to do it.
The system make adjustments to the player's price according to the variation of his skills, age, position, etc. That appraisal is used as reference price for the calculation of the highest and lowest price you can ask/pay for that player.
On the other side, the price of the player is defined by the clause specified in the contract. If another club pays the full clause for any of your players, that player will leave your team and immediately will join his new club, and there is nothing you can do to keep him. For this reason is critically important to keep your best players' clauses high enough so they will prevent other teams to paye them (or if they choose to do, to get a good amount of cash for them), but at a reasonable level for the rest.
When you are about to make a bid for a given player, you will have to adjust not only how much money you offer to the other club but also how much you offer to the player and the duration of the contracts. The player can accept the offer or reject it according to his own priorities.
Otherwise, if you don't want to pay the full clause, you can always make a bid and try to negotiate its price. The estimated value of the player limits the amount you can offer for the player (or how much can other clubs offer for your players). You can only make bids for a player if he is marked as available for selling.
If you have not money enough to sign up the players your team needs, you have the option of getting some players on loan so you have to play only their wage (the image rights from these players will be awarded to their club, not to yours). Bear in mind that those players that are in the last year of their contract cannot be given on loan to other clubs.
After the end of a league you will have to renew your players' contracts if you did not it already. Those players with just one year of contract remaining will go to the free market if they are not renewed prior to the next reset of the league, so you will lose them. These players appears with a red mark at your team page.
In general terms, young players (those younger than 25 y.o.) will ask you for a increase in their wages with each renewal. From 25 y.o. on they will be glad with the current wage and from 30 y.o. they will even accept a reasonable reduction of their income.
-The coaches also have a contract with the corresponding wage and clause, but in this case you can only sign a new coach paying the full clause.
-Try to negotiate via messages for those players that are available for lending or selling from their clubs. To fail to do it is considered bad manners.
-Free players can only be signed paying the full clause. They are suposed to play in minor or foreing leagues that are not part from virtuaManager.