First of all, never buy accounts, take out all the armor money etc... transfer to yours, he will get it back no matter what because he is the owner, even if he failed the first time, he will keep coming right back for his account, when all the valuable stuff is onto your account, fuck up the account, use some very detectable bots and do some really pointless stuff, like if you woodcut, then transfer it to your account or just drop all the wood, always swear, talk about stuff, ask little kids for their information, advertise websites, get it perm banned or perm mute, because you won't have that account for long, why not make it that nobody will have it since you paid for it!
EDIT: Since you know his IP if you live close to him, go pay him a little visit, if you are sure you can't take him down yourself, call up some friends, or bring a baseball bat etc.. teach him a lesson, so beat the crap out of him, and take back your money, and fuck his account up!