If you've recently purchased a fake handbag online (ebay, bonanza, etc) please read this thread for instructions on how to proceed. This is just advice and is in no way a guarantee that you will win your claim. Should you have any additional, specific questions please start a new thread.
#1) Contact your seller to see if they will freely allow you to return your item for a complete refund. Sometimes sellers will not put up a fight. Please note that if they do allow you to return the bag, the return shipping will likely come out of your own pocket. Also, for your own protection I suggest getting signature confirmation and insurance, especially if a lot of money is on the line.
#2) What to do if your seller won't accept a return? Open an ebay or paypal claim immediately. You don't need to escalate it yet but opening a claim will let your seller know you're serious and will alert ebay/ paypal to the issue.
* If you purchased your item thru ebay, file an ebay claim to utilize "ebay buyer protection".
* If you did not go through ebay, but paid through paypal, file your claim with paypal to take advantage of paypal's buyer protection.
#3) Escalate the claim once you're ready to make your case. You should not need a written letter of authenticity/ non-authenticity if you file your claim through ebay but you may need one for a paypal claim to prove non-authenticity. If a written letter of authenticity is needed for any reason, see below for more information.
#4) If all else fails and ebay/ paypal are of no help, try a credit card chargeback. Of course you'd have to have paid with a credit card in order to do so. If you've paid with western union, bank wire transfer or paypal balance, you're options are very limited unfortunately but still worth a try.