Allow me to define thread-crapping:
Joe: I do paint jobs for $15.00.
Jay: Joe sucks! I do them for $10.00! PM me!
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Allow me to define the consequences of BAD BUSINESS:
Joe: I do paint jobs for $15.00.
Jim: Hi, I had a bad experience with Joe, and rather than remedy the situation, he told me to leave him alone. Just letting any potential buyers know that there is potential risk involved.
John: Thanks, Jim. Sorry, Joe, I'll buy from someone else.
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In case I'm not clear, if you have had a bad experience with a seller and have reasonably justifiable reasons for your stance, you are allowed to voice this.