OK -- so, having been duped twice, and now having bought an AUTHENTIC Balenciaga bag from Harvey Nichols here in London, I am here to share some advice with you.
The thing is this: as far as my experience had gone, I had never seen an authentic Balenciaga before I bought my first fake. There aren't any Balenciaga boutiques in my city, and as you may have seen, Balenciaga isn't sold online at reliable department stores. I actually bought my first fake from one of those discount luxury handbag websites. I thought I was getting a huge deal. Ha! It actually was quite an accurate replica though.  The fakers do use an extremely similar dust bag like the one that comes with the authentic. Good one, guys! And since I had no clue about what a real Balenciaga was like, I was satisfied.... until....
Six months or so later, the bag started falling apart. I paid $600.00 for this fake, mind you. The first thing that happened was that the silver plate inside (yes, even the fakes come with an engraved silver Balenciaga plate inside with a serial number) came off. It was very loosely held in place with threads to begin with. This happened with my second fake as well.  The zipper became twisted and difficult to close. And yes, it was a Lampo zipper -- fakers use those too! Then, two of the studs that hold the shoulder strap to the bag came off; fortunately I found one and managed to screw it back on as well as I could. Then there were other little details that just seemed wrong, little flaws that I knew shouldn't have been there. As much as I didn't want to believe it, I knew I had a fake.
I got my second fake off of ebay. The new fake absoluetly reeked of leather -- a sign of cheap leather -- and the threads holding the silver plate literally fell apart the moment I went to examine it. Something like that is a dead giveaway. There is no way that an extremely expensive luxury designer like Balenciaga would ever have anything but flawless craftsmanship.
So, I was very excited to finally see real authentic Balenciagas firsthand. I was dying to see the differences! I am sad to say though, there weren't many. Due to the popularity of this bag, the fakers have had many opportunities to hone their craft. I am not going to go into details, because I am not about to help the counterfeit crooks make better fakes... and anyway, Balenciaga routinely makes minor changes each time around in the hardware color, engravings etc. But I will point out a few things:

1.  First of all, an AUTHENTIC Balenciaga bag in the SMALL size costs about $1,200.00.  So all you people eyeing the """Balenciaga""" bags selling on ebay for $200.00 - $300.00 and thinking what a great deal (especially being sold by someone with 0 feedback or 15 feedback or something ridiculous like that) -- THINK ABOUT IT!!! Who is going to sell, or even be able to obtain to sell, bags that cost this much money for less than a quarter of the price?? It doesn't make any sense.  What does make sense is selling a cheap $20.00 handbag for $300.00, at least from the seller's point of view.

2.  Now that you know the above with regards to the price... if you're going to pay full price, why on earth do it on ebay? SCAMMERS ON EBAY SELL FAKE ITEMS FOR THE FULL PRICE ALL THE TIME. How much would that hurt? If you're going to do the smart thing and fork over the full authentic price, go to Balenciaga's website, look up the boutiques that sell them, call them up and ask if you can place an order with them for them to ship to you.  They would most likely be happy to help you, and you will have THE REAL THING.  Don't go to ebay for these things.  EBAY IS A SCAMMER'S PARADISE. 

3.  One thing the fakers can't fake is the quality of the leather. An authentic Balenciaga is made of gorgeous, buttery soft leather.  You can tell the moment you lay your hands on it.  The fake Balenciagas are made of tougher, cheaper leather.

4.  The reason so many buyers selling these """Balenciaga""" bags on ebay have 0 feedback is because they keep opening new accounts to sell their fake bags. It would look suspicious to see a seller who has sold many $300.00 """Balenciaga""" bags.

5.  The "controllato card" that the fakers love to crow about in their auctions means absolutely nothing.  My fakes came with controllato cards too.

6.  Providing a receipt means absolutely nothing as well. Theoretically, I could take my receipt and papers from the Balenciaga I just purchased, stick them in a fake Balenciaga and make the same claims all these other fakers are, and rip people off.

I hope this guide helps someone. I know at the time I bought my fakes, there were no Balenciaga guides consult. DON'T SUPPORT COUNTERFEIT CROOKS! BUY YOUR LUXURY GOODS FROM AUTHORIZED LEGITIMATE BOUTIQUES AND LUXURY DEPARTMENT STORES!