It seems that there are so many scam artists out there. When you read an auction what factors help you decide that its ok to purchase from a seller?
1. Pictures - are there many pictures? Are they clear or do they look like they're trying to 'hide' something? For example, and i'm not a paddington expert or anything, but the keyhole has to face a certain way. If you find that the pictures seem to be intentionally blocking the keyhole, i'd say they're trying to hide something. Also, do a bit of research on the item, go on the Authenticate this forums and see what are the critical authenticating parts of the item. Scammers will try to photograph away from these areas. Some scamming auctions also use stolen pictures - so always ask for more pictures (more than one), especially from obscure angles in case they are using stolen pictures. Stay away from auctions with stock photos - the ones used on bluefly or NM for example.
2. Communication - email them, see if they reply in a rational, helpful and prompt manner. Usually the very communicative ones who go out of the way to send photos and help you in anyway are not scammers.
3. I've found that country of origin or how well the auction is written in terms of English has no bearing on the item's authenticity or the seller's. Some people with horrible English have sold beautiful items, and some people with great English are scammers. Country of origin is definitely something people would ask you to look out for, for example some people wouldn't buy from China, but having lived in China for a month once and selling on ebay I can tell you that I was a legitimate seller. It comes down to the pictures and the seller's attitude.
4. Feedback - always go through toolhaus.org to read their negatives. Under 10 feedbacks I wouldn't buy unless they've gone out of their way to help me, and it also depends on what they've sold before.
I still think the best way to see if someone will be scamming you or not is to ask for lots of photos because:
a) You'll find out if they have the bag.
b) You'll know their attitude towards helping you out.
c) You'll know they actually have the bag to give to you (ie not using stolen pictures).
(Those picture tips were given to me from a friend and she saved me from buying a fake bag off the internet.)
Even with that though, it comes down to a gut feeling. If the person just seems untrustworthy, don't bid. Always do your homework beforehand.
Oops I just realised I didn't answer your question which was how do you tell in the auction? The truth is, you can't tell. Some people are so sweet in their wording and then bam! No bag, no money. You really have to dig around for more info about the seller and their attitude for those high priced items.
One more thing, if a seller is selling multiple quantities of the same item, beware if they're luxury items like handbags. Check their previous sales to see if they've sold more of the same exact item. If so, you may be dealing with someone who is selling counterfeits.