With trends changing fast and inflation on the rise, you really want to get a genuine good and also be not ripped off. Ebay have a great selection of clothes, but it's eliminating the ''fake's'', so you'll be completely satisfied with your goods.
Firstly, Let's start with taking a look at the price of the good. Typical designer clothes arn't going to be extremely cheap, unless they've been worn and or damaged goods.I'd say a fair price is anything 15.00 upwards, but of course, there are some genuine's under this price, it's just a rough scale.
Secondly, have a look at where the goods are coming from, is it from Asia or somewhere similar? Are they selling in bulk quantities? Ask yourself these questions before placing a bid.
Take a look at the sellers feedback, see if it says anything about ''fake'' goods, don't just see their % and be happy. Have a look at what other stuff their selling.
A common mistake by some people, make sure it says 100% authentic, in the description, before purchasing (unless of course you wan't a fake ;p)
Now, take a VERY close look at the pictures they provide, are the pictures blurry? If the designer usually has an emblem, or logo, match it with an orginal on the internet. Do NOT hesistate to contact the buyer and ask him about the good, (label's e.t.c)
Consider all of the above before making a purchase, and i'm certain you will be happy with your goods.