Trade is an important part of the road to success. If you use some time finding the right players at the right prices you can earn a good amount of extra Credits when reselling them.
To buy a player you must find him first. You can find available players on auction, the transfer list and on free transfer. You can find these three search options by going to "Market" and check the sidebar on the left.
When you have found a player you want to buy or bid on you can press the "Place a bid" button, or if its a player from the free transfer list, you click the "Sign on free transfer" button.
When you place a bid you must define a "bid" and an "auto bid".
Bid is the amount of Credits you want to bid for the player. Auto bid in the maximum amount of Credits you want to spend on the player.
If your bid is accepted an auction starts, that you will lead with your bid, unless another player has a Standing Bid on the player. Should another club place a bid that is higher than your bid but less than your auto bid the system will automatically outbid that club for you. Your auto bid is hidden from everyone else.
You can monitor your auctions by going to Market > Buying > Auctions. On this page you can also increase (or decrease) your auto bid and monitor the amount of time there is left of the auction.
Standing bid:
A standing bid works like an auto bid, but still a bit different, as the current owner of the player does not get notified, nor do they have a bid to accept or reject. No auction will begin at that point, but you will have a bid on the player in case an auction is started by someone else - then the system will automatically put your standing bid towards the auction.
It can also be useful to put a bid on different players, without tying up your credits and squad places.
Example of an auction:
You bid 50.000 Credits on a player and put your auto bid at 75.000 Credits.
Another club bids 55.000 Credits. The system will automatically bid 55.001 Credits for you, as the current bid (55.000) is lower than your auto bid (75.000).
Another club then bids 90,000 Credits. The system will register that bid at 75,001 Credits in their favour, until you or anyone else may bid more, etc.
You can also use that "Raise bid" button to decrease your auto bid if you've changed your mind on wanting the player for any reason - to 1 Credit above the current highest bid - and then hope you get outbid more easily.