Theres plenty of technology that filters out proxies/vpns. No matter how hard you try you wont be able to access the forum with vpn/proxy. I can list you at least a dozen of forums that you wont be able to access it with vpn. It will either forbid you access or just tell you the site is offline. Doesn't matter. And I'm not talking about some free chrome extensions/tor or whatever. Paid vpn.
About the unique IP. Lets say some member registers multiple accounts on the same IP. It would be nice if we check on his account profile, it would tell us how many matching IPs he has with another members. Has more than 1? This user possibly has one legit account, and is scamming with the others.
And I see your point about the account sales, keeping its members on this forum and not elsewhere..
So we don't allow people to use proxies at all? Why not? Many of the Staff use them. So we punish users who use them to protect their privacy or security just to stop a few scammers with alt accounts? Whatever restrictions you put in place they can just as easily be bypassed and punish legit users in the first place. You don't even need to use proxies either. Just use whatever wi-fi signal you can pick up or your mobile internet. Get a few different pay as you go phones and use the internet connections of each one. Also, what happens if two people happen to log on from the same Mcdonalds one time? It now looks like he has alt accounts. It just adds more trouble.