Hm. OK, let me put this another way. As you browse the messages on this particular forum, one thing will stand out to you, as it does to me.
There are many, many people selling merchandise on eBay that is fraudulently labelled. Some sellers may be aware of this, others may not be.
But that is really irrelevant, because if what you wish is to purchase a bag that is produced by a specific company, unless you have such an advanced level of knowledge and familiarity not only with the bag itself, but that a specific seller does indeed sell only truthfully labelled merchandise, then eBay is not, in my opinion, a good choice for you.
In the absence of such expertise, which your message indicates is indeed absent, the most reliable way to be sure that you are obtaining an item that was indeed made by a particular company is to purchase it either directly from the company's website, or from an authorized reseller of the company's products. The company itself is the best source for obtaining a list of authorized resellers.
For a variety of reasons, eBay has become an increasingly risky proposition for both buyers and sellers alike, and not just with regard to handbags.
It is for these reasons that I recommend that eBay be used to purchase inexpensive items only, and that for high-ticket articles, such as "designer" handbags, the actual company, or as close as you can get to it, i.e. authorized resellers, are a better option.
With regard to "cheap" fraudulently labelled bags, even completely up-front and legal lookalike bags, as someone pointed out the other day, it is possible to obtain fraudulently labelled bags for very little money - just not on eBay.
And while I am by no means an expert on designer bags, quite the contrary, I know less about them than anybody else here, but I can tell you that I have seen bags that look exactly like one made by a particular designer, NOT fraudulently labelled, making no claims to be the product of the designer's company, sold for under $15 in Marshall's or the Burlington Coat Factory, and I have seen that very same bag, again, neither the designer's product nor claiming to be, but on eBay that same bag will be sold for over ten times that amount.
And as for the genuine designer article, the people who sell those on eBay are very rarely going to sell them for next to nothing. Under retail, yes. Especially in the case of used bags. That's another thing I have learned here. It appears to be a fairly popular practice to buy and sell used designer bags. And there are people who do it on eBay. But I don't think that eBay is the place to look for designer handbag bargains, and I don't think designer handbags are the most likely category of merchandise for bargain-hunters anywhere.
Therefore the best "trick" for avoiding the purchase of a fraudulently labelled bag on eBay is - don't use eBay to buy bags - or any other high dollar item that anyone would be likely to fraudulently label!