(1) Check seller lists at mypoupette and ALVA (
http://omgoddess.ms11.net/sellerselect). This is NOT foolproof, so you can't "just trust" the sellers, but it's a good start.
(2) Get the bag authenticated on the forum. I recommend you do some studying of other "authenticate this" posts for the same bag you want, so that you can see what additional pictures you will need. Then you can collect the additional pictures from the seller and post those ALONG WITH the original auction link on the "authenticate this" thread. It saves a lot of time and also is considerate to the authenticators
(3) Check to see if you get PayPal protection, and whether the limit is $300 or $2000 (for instance, international transactions often do not qualify for the higher protection limit). There should be a "PayPal protection" icon at the right side of the screen if the auction is eligible.
(4) Check seller feedback carefully. FB can be faked, so look at who's posting it: do they all have 1 or 2 FB or do they have all varying amounts? Is the FB all very recent? Is it all for very cheap or very similar items, or more varied?
(5) Pay through PayPal, with a credit card.
(6) BEFORE leaving FB, take the bag to a LV retailer and ask to speak to a manager. Ask whether it's a real bag. Once you have gotten confirmation that the bag is real, only then should you leave positive FB.
(7) Finally, PLEASE BE CONSIDERATE OF THE SELLER. This will mean several things for you, as a new buyer. First, you need to keep in mind that a seller is taking a risk by selling to you. The seller probably has a financial investment in the bag. So if you turn out to be a scammer, not only is she out the cash you refused to pay her (or charged-back to your CC undeservedly), but she's also out a huge financial outlay for her inventory. She is right to be suspicious of you, so PLEASE be POLITE and COMMUNICATIVE. Get your PayPal WITH CONFIRMED ADDRESS set up BEFORE you bid. Contact the seller before bidding--just send a short note saying, "hi, I'm new to eBay, I really like this bag. If I win the auction, I will be paying immediately with AmEx/Discover/Visa/etc. through PayPal. I have a confirmed address, which is where I would ask you to ship the item. Please let me know if there's anything else you need to know!" Then if you do win, PAY IMMEDIATELY. NO EXCUSES. Be consistently polite and communicative--if the seller has follow up emails, answer them promptly. Then once you have confirmed the bag is real, PLEASE leave USEFUL positive FB. That means saying "LV wallet, as described, authenticated at Short Hills LV. Fast ship. A+" DO NOT just put something like "Thanks!"--that does NOT help future buyers, and it does NOT help the seller get more business.
Overall, you will find that buying authentic bags on eBay is time consuming. I find that to just be a fact you need to live with. If you are not OK with putting serious time and effort in, then you should just save up a little longer and buy new from the boutique. Good luck to you!
PS: There's always let-trade.com