It's a valid concern since this is an area of the Fediverse that requires more oversight for safety, and smaller instances may not have the personnel for it.
I think it can still work, but something like this should be structured so that:
- People moderate local groups, similar to how communities work on Lemmy/Piefed/Mbin
- You join the group that is local to your region, or to your hobby
That way it isn't on the admins to look over everything, and moderators can take action on what they have experience with.
Past that, the usual rules apply when buying something from people online:
- meet up in person, unless they person's profile makes you trust them
- meet in a public place, even better if it is a place that is designated for such meetups
- don't pay them online before you have seen and tested the item
Reminded me to check on this Kickstarter I saw ages ago
https://www.kickstarter.com/projects/solidfactory/the-cup-a-bug-a-no-kill-bug-catcher-by-solid-factory/description
Looks like they made it, I might grab one at some point
https://cupabug.com/product/cab/