Whatever it is Trademe are making assumptions with IP addresses that should not be made.
don't take this personally, but i think you're making the assumption that TradeMe is making that assumption based solely on IP addresses. You have presented no proof to support that. Yes, they cancelled your account, but have you confirmed that it's really just because of the IP address?
even if the whole world completely and absolutely adopted IPv6 for every single device, i can still check my TM account on my wife's/partner's/gf's/mate's device - leading to the same issue.
There will be up to 100 people accessing TradeMe from a single IP address at my workplace. Not a problem of this nature. There are far bigger workplaces (headcount) in the country.