I don't believe that any of the suggestions laid out so far do anything to either prevent scams nor mitigate the effect of one should it happen. Let's scrutinize, shall we?
Organizers should be limited to 1 group buy at any given time, and required to complete it before moving on to another.
ddot makes good points about this rule above. I'll make some of my own and reinforce some of his.
First, the rule isn't enforceable. Just because overlapping group buys might not be conducted on geekhack, they can still be conducted elsewhere: Reddit, DT, etc. Sure, you can prevent multiple active group buys from the same organizer in the Group Buys subforum, but you can't prevent someone from posting about other buys in a different subforum – this happens all the time already (Chinese buys as an example). Sufficiently accessible group buys run not on geekhack will still be joined by members of our community.
Second, this rule doesn't guarantee any mitigation of the impact of a scam. Sure, it's one buy, but we all know how huge buys can get. Someone set on taking advantage of the community still has every means to do so, and is not meaningfully limited by this rule.
Third, it has the significant drawback of limiting output from organizers willing and able to take on multiple buys simultaneously.
Require regularly scheduled updates.
Good group buy leaders already give regular updates. If anything, a lack of updates is a useful signal that something might be wrong with a buy. Whether or not people have regular updates does literally nothing to expedite the process – it just makes you feel a little better. I'd go as far as to say this rule is harmful, because it would require that organizers looking to take advantage give falsified updates to placate people. Silence is loud: use it as an indicator that something might be up.
Require posted proof of payment to the manufacturer.
This is easily falsified. Sure, we can implement this if it makes people feel better, but that's literally all it does – make naive people feel better.
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Finally, I believe that geekhack has and should have no responsibility in how group buys are conducted. This is the responsibility of the participant. Each and every person going in on a group buy should understand that there is a chance that nothing comes to fruition, either at the fault of the organizer or for other reasons. If this is not an acceptable possibility to someone, they should not participate.
The only idea submitted thus far that I can get behind is a more prominently placed notice/warning above every group buy thread. Having a sticky in the subforum is not sufficient; people don't even read the entire OP of a buy, much less a separate thread.