have you bought off ebay at all before? the only thing knitting related i have bought off there so far is my Boye's Interchangeables (woo hoo!) but i am pretty careful about checking over those auctions no matter what i buy and checking the feedback of the seller (including making sure all that wonderful positive feedback isn't just from their own buying...) as of yet, i have been pretty lucky and haven't lost any money on Ebay but i think it is because i only buy from well established sellers...no matter how much i want an item (and there are well established ones i won't buy from if their info is contradictory even if it appears to be an oversite...just don't wanna lose good yarn buyin' money ya know!
as far as anything specific to yarn that i don't know and would love to hear it. I see some beautiful stuff on there too but i can't help but feel like i am missing the touch and feel experience that i get from going to my LYS...but that hand spun/painted stuff calls to me in my sleep i tell ya...lol