In my case it would be shill bidding and related practices. I think shill bidding has the biggest negative impact on the hobby since it artificially inflates values, and hurts both buyers and sellers.
Each of the other things are, in my opinion, bad. However, each is also somewhat mitigated by knowledge. The more I know about a given issue, or about the hobby in general, the less likely I am to be victimized by any of those things. And while I've certainly encountered each of them (except #5), I think it's #3 that I can protect myself against the LEAST.
-Al
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