The salary is different for each team, its the amount of money (per year) that your team is allowed to pay its players and coaches.
There is no "selling" of players. You can release a player from your team, but that persons salary will be counted against your salary cap until another team pick up that player (through waivers), or until his contract with you runs out. For instance, if Player A is on contract with your team at a salary of 4 million a year for 3 years, and you realease him from your team, his 4 million will still count against your cap for the next 3 years. If another team picks him up after you do, your team is released of the contract and the other team is now responsible.
You can "buy" = sign a free agent, but in order to do so, you have to have room "under" your salary cap. If a player you want costs more than the space you have, you cant sign that player. The only ways a team can sign a free agent when they are over the cap is if you sign the player to his minimum salary, or with a mid-level exception. you will see if you have an available exception when you are searching the free agent pool.
Hopefully this gives you a little to work with, I'm not the best at explaining, and there is more stuff to consider but i though a general overview will help yo uthe best. after getting used to these, you will learn how to best use your exception and and sign/release players.
Let me know if I can help you more, I'll do the best I can.
