I agree with you that it is counterproductive to explicitly allow account sales, but your proposed method of identifying them won't work:
For those that continue trading forum accounts despite a rule prohibiting such trading, to confirm that a person is in fact selling accounts, they could look at IP addresses/browser fingerprints of the person, and when they see the account being used by a distinct second person (with other accounts) with a different IP address/browser fingerprint, the administration will know the person is selling accounts.
One person may access the forum from several different IPs (home, mobile, work, public, etc.), use different VPN servers, or use Tor. One person may also user several browsers across different devices (or even on the same device) and across different operating systems, or even use a switcher or spoofer to constantly display a different browser fingerprint. Even if they didn't, account buyers could simply use the defense "I switched to a new browser/OS/computer/laptop/phone/mobile provider/WiFi provider/VPN provider/Tor".
Even if your proposed method would work, it would put an impossible workload on the admins since they are the only ones with access to IP logs, and I can't imagine theymos would be very keen to give this privilege to anyone else short of perhaps one or two of the mods.