I strongly recommend the use of the feedback features of both applications CL and EBay
If a person asks me to "outbid" someone else, I let them know that their email reply has been forwarded to the CL abuse group
http://sfbay.craigslist.org/feedback
Same with Ebay, if a seller tries to "end around" Ebay I report them.
I figure if they are willing to cheat Ebay, why not me too?
I agree, if a person has listed a sewing machine with a low shipping fee, I message them and ask them to describe how they plan to pack it. You would be AMAZED what I have heard back... from "I will just wrap it in a couple of towels" to and "I just put it in a box and ship it" all the way to "I pack it so well it adds 3 pounds to the shipment"
I send them instructions on how to pack a sewing machine and ask for their reply, then I gauge their reaction to determine whether to bid or not.