I guess you need to change some stuff in the sourcecode for that. See if someone with a connection that uses that IP/MAC allready used that command. But there is no real way to prevent such things, since there are Proxys/VPNs or MACspoofers that simply make that kind of countermeasures useless.
What is the purpose of this? Do you want to allow multi accounting and every player to have a offline shop?
On the other hand, if you for example have 2 siblings playing in the same household and both want to sell offline while doing something else only one can.