Ohhhh ...
well, i will try to deal with your "request".
first: if it would be SO easy to steal Account information from other People, there wouldnt be any MMORPGs out there. So as you already can think of: there are (surly) protection system in some games - for example: in Lineage 2 there exist some kind of "Key Security" which is (default) deactivated.
second: EVEN if you manage to code a Keylogger, which is be able to collect and send you the keys over the internet, you still have the problem to "infect" "possible" targets. You also have to think about that some (or if you a newbie) the most of your "own" coded keyloggers will be found by even freeware Virusscanners.
third: If the unbeliveable state appear that you managed both problems above, and you've got a legit, running (not deactivated) username and account of someone who is playing your MMORPG (in this case Lineage 2), you have to log into his account and "steal" the Items. If you think now you can only log in and "trade" the target items to your account you should rethink it.
Because your Target wont do nothing if it finds out he was hacked, you have to be extra aware of GameMaster Banning AND law problems! Because every item in Lineage 2 has an unique ID (i dont know if this is really true) its easy for them to find out who it already has or (over the logging function) to see who had it in the past. Or to say it short, its tracable.
So IF I would be a GameMaster / Support worker of Lineage 2 - i would check which IP had your target while he "traded/dropped" his items to you. If there would be a unusual IP, for example, he normaly log in from USA, and only for this time over a Proxy (yes there are lists over lists for Proxies) or directly from a european state, i would be really really sceptic and contact the internal law organisation. Also if the IPs from Target and Traded Person are the same.
So YOU have to overthink the whole process. To "perfectly" hack someone, and get his itmes, w/o any law problems (in the future), you have to code a keylogger which is be able to destroy his self without ANY TRACES on the target PC. ALSO any E-Mails / Internet Pages ect. which could lead the way to you! But this means also you need a high Coding skill (from you), to delete the System log, delete the E-Mail of the target System (if you infected him over E-Mail), delete the files with a high sensitive destroy (RAM only) system. ect.
now you think - okay, thats all ? No surly - we have still the IP Problem. Even if you find out which IP your target usualy have (its relative easy), it would be the best if he gives you freely the items. therefore you need a bot, on the target system, which logges into the game, make a trade shop (title: i will quit) and you buy it directly from him - so you wont have any problems
I KNOW, you can ALWAYS tell he gave you freely his Account informations, and there would be everytime a 1vs1 Law discussion which you will surely win, but if this happens more then only once you will get fast problems.