As a seller of donated goods on eBay, one of my greatest fears is that an item that I've listed is a fake.  We all know that listing counterfeit, fake or questionable items on eBay can get you into trouble.  Listings pulled, account suspended, a returned items and bad feedback.  All of these will cost you as a seller.  Are there ways you can know that an item is authentic and not counterfeit?  The unfortunate fact of the matter is; unless you bought the item from a reputable dealer,  you can't.  Nevertheless, there are steps you can take to avoid listing counterfeits. 
This guide will focus on designer jeans.

Primarily, go with your instincts.  If you get the feeling that something isn’t right, pass on it.  There will always be more and your gut is your best friend at times like that.

Do some research and learn what your product looks like.  Go to brick and mortar stores, check details and ask questions.  Look at buttons, tags, seams, embellishments, cuffs, waistbands, and logos.  All of these are important indicators of authenticity and must be scrutinized without mercy.  After asking the store staff, try to get some photos of the tags and embellishments, so you will know what to look for later.  Tell them who you are and what you are doing BEFORE taking photos.  The store’s owner probably hates counterfeits as much as we do.   

If you aren’t too shy, stop people on the street and ask them where they got their jeans.  Most will be flattered and tell you all about it.

There are websites out there that solely discuss the authenticity of jeans being sold on eBay.  The people that run these sites and the people that post on them are diligent and helpful.  They want to keep the "jean pool” clean, as it were.  Honestforum dot com is my favorite one for this, but there are many.

Check other eBay auctions to see what is selling and compare your product.  I cannot say enough, look at tags, buttons, zippers, seams and embellishments.  If anything in your product seems off, discard it.  It is not worth your eBay reputation to take the risk.

I hope this guide was helpful and Happy eBaying!