There's no way to stop scammers from scamming people, but doing nothing is not improving situation at all, allowing to people sell forum accounts and allowing inactive accounts is good ground for scammers. Of course there are people really good in scamming and this will not stop them, but most part of scammers are kids or just idiots. Inactive member list cleaning and controlling clones is not so hard nowadays(there is a lot of forum plugins), but will save bunch of not so advanced people from being scammed.
In every "profession"(i don't know how to call scamming) are newbies/kids and professionals. You can't fight professionals(maybe professional can fight professional, but professional always will cost money), but you can fight newbies/kids for sure and keep your community safe.
All that is just IMHO and based on my experience administrating forums with marketplaces. And doing all these little things helped people to save money. The number of scammed users dropped.
I don't believe it's preferable to prevent people from getting scammed; this incubates suckers. You're simply giving the community the fish and increasing their reliance on you to do anything. Rather, people should be exposed to scammers, all kinds of scammers, so they'll know who to trust and who not to trust on their own, which will be a skill they'll keep forever. You may want to protect us from the "kiddies" and the "idiots", but if we can't even protect ourselves from them, then what does that say of the community? What's below an idiot? Apparently, "not so advanced people"