Ok there is more than one way of trading ISK in game, albeit some more risky than others (due to ISK farmers). You can simply send someone the ISK, or contract it out to them. Ill explain each way in detail in case others do not know how.
1. Direct Payment
To simply send someone the ISK, you will need to open up your People and Places menu (the magnifying glass with little people icon on the left side of the screen). At the top, there is a search drop down menu - select Character from this list. In the text box next to it, type the character's name that they wish the money to be sent to. MAKE SURE YOU TYPE IN THE CORRECT NAME!!! There could be many people with similar names, so select the right one!!! When you see the desired character, right click their portrait. A list will pop up - select "give money." Type in the agreed amount, and click OK. Oh, another thing - make up some bs reason for sending the ISK, such as "ransom money." This will throw off the scent that the transaction isnt legit, as is the case with most ISK farmers, whose transactions get reversed A LOT. This will just add security to the transaction and make both parties happy
2. Contract Payment (note: there will be a fee for creating the contract. ~10,000 ISK)
A little more complex to those who arent familiar with the contracting system, but more safer than simply sending the ISK themselves. To do this, open up the Contracts menu (the piece of paper with a hand writing on it icon on the left side of the screen). Choose to create a new contract. For contract type, chose "Item Exchange", and for Availabilty, choose "Private". Type in the character's name you will be sending the ISK to in the provided text box. MAKE SURE YOURE 100% CORRECT WITH THE SPELLING!!! Click next when all this has been completed. It will ask you to select some items - ignore this step and click next. In the "I Will Pay" text box, type in the agreed upon amount of ISK. For the expiration amount, choose 3 days, unless otherwise notified, as not everyone on this board is on at the same time. This will allow ample time for the person to accept the contract
Click next, then finish. One last box will pop up (unless youve already made a contract before and chose "do not ask me again) - click yes. All set!!!
Now, to accept the contract, there should be a contract displaye to you. Click Accept, then yes. VOILA!!! If not, you will notice your Contracts icon flashing, or lit up. CLick on it. You will notice that there is a contract available to you - YAY!!! Click on it. Double click on the contract issued to you by the other party, then click on Accept. Finally, click Yes, and youll be all set!
Hope this helps people out with the whole sending/receiving ISK through in game means