buying has been fixed (obviously) in the server, it only works on older ones. What you CAN try to buy is things you are not supposed to have (like 30.000 patk dagger) of course those will get you banned in no time.
In newer servers buying things not in the buylist (if you see the packets when you buy you will see this one) will get you either kicked or sent to jail. So even that does not work.
There are other things packethack can be used for but it varies from server to server, try it out (dont be stupid and create a NEW account, and use a free proxy or vpn server to test it) capture packets, experiment re-sending packets to the server, change the order of them ... in other words play a little bit with it its a very powerful tool.
This is the kind of tool from where other tools/exploits are made, and its not for everyone. You can make a radar for seeing other ppl. You can make a script auto-targeting a rival clan. But you will need some programming knowledge ... fo the simpler things you need common sense and luck