Just a word of warning. Credit/debit card buyer protection only works for official transactions. This means from online merchants, or auctions that did *not* end prematurely, as in your case. What this means is, do NOT transact outside of the auction unless the guy has a lot of positive feedback. If you can, ask people in his feedback history for confirmation of his integrity. I find the eBay feedback system is a pretty reliable indicator, anyone with 20+ *recent* ones is a pretty good bet. Too far in the past and the guy could be back for one big con, or it could have been hacked into. Also check if the item is in line with his normal wares - for eg, an eBay camera shop suddenly selling jewellery is reason to be suspicious.
Yahoo auctions are a bit more risky, simply because unlike eBay it's free to list and I'm sure people take advantage of that.
Paypal protection only works IF the guy has enough money in his account to reclaim at point of investigation. If he's taken your money and run, which is very likely given he's a conman and knows his tricks, you're left with your pants figuratively around your ankles.
I lost US$100 once to a fraudulent eBay seller, Paypal wouldn't repay me for reasons mentioned above, but UOB debit card protection came through with flying colours.
Never (ever ever ever!) pay using Western Union, eEscrow, etc, no matter what the hell they say, no matter how high their feedback is. Absolutely never. If they do that much online business, I guarantee you that they should have a proper credit card facility or accept Paypal.
Hope this helps for future occasions, but it looks like, sadly, this is as you say a lesson learnt for which you won't be able to recover the cash. Just wondering, how much did your sister lose?