The transfer market can be overwhelming in the beginning, but you will be raiding the market in no time and will be a top class manager with enough practice. You can also find tutorials on YouTube
On the transfer market you can buy new players or sell players from your squad. Here you will always find between 10 and 20 of the total of over 500 Bundesliga players offered by the transfer market. There are daily and randomly 5-10 new players on the transfer market and daily about 5-10 players expire. This creates an ongoing rotation. The length of stay of the players offered on the transfer market varies.
If several managers bid on a player, the manager with the highest bid gets the player. Bids are made face down, i.e. you cannot see if other managers are bidding on a player.
Bids that are below the market value of the player are sometimes rejected by the transfer market. If two managers bid the same amount, the manager who placed his bid first gets the player.
Each player has an expiration time. You can find this under the player's picture. When this time expires, the player moves to the highest bidding manager or goes back to the transfer market.
If you place one of your players on the transfer market, you will receive an offer from the transfer market within one hour. Of course, your teammates can also make you an offer. You choose which offer you want to accept.
On the transfer market you will see players that have been put on the transfer market by other managers in addition to the players offered by the transfer market. If you bid on one of these players, the respective manager can either accept or reject your offer.
In challenge mode there is no transfer market. You can add and remove any player from the lineup. Just make sure that you stay in budget and that your account balance doesn't slip into the minus.