1) Price tells everything - Although the average price on eBay is lower than the market price, still be aware to those extremely low price items. For example, a pair of new Air Jordan shoes costs 300 dollars on the market, if you see a pair on eBay that is like 40, 50 dollars, I can almost guarantee you they are cheap replicas.
2) Watch out for the high risk items - Those are the brand names often being replicated. So far, I have found those replica brands on eBay, UGG, Nike, Puma, Adidas, Abercrombie & Fitch, North Face, Spyder, Lacoste, Ralph Lauren, BAPE, LV, Coach, and etc. You just have to pay extra cautious when you buy those items.
3) Feedback, feedback - Read their feedback, even if they have 100% feedbacks. Some time, a buyer will leave a positive feedback even if they know the item they are buying is fake or they don't even realize that. Read those feedback carefully because some buyer will say something like the item is not from USA. Which is a clear sign to say it is not an authentic item when you are buying America brand names.
4) Check out the User Satisfaction Rating - Those rating indicates a seller's performance. Usually, a foreign replica seller will receive low rating on "shipping time" and "item as described."
5) Don't be fooled by seller's country - Some of the foreign replica seller register with an fake US address, so their account will look like it is US based.
6) Shipping method - Foreign replica sellers usually use EMS (Express Mail Service) to ship their item or Air Mail, US sellers use USPS/FedEx/UPS and other common carriers.
7) Ask Question - Ask seller question about item's authenticity. If a seller does not reply or go around with it is most likely a fake item.
8) English Usage - This is kind funny but often time if you read those foreign replica seller's item description, you see very poor English grammar. Obviously that can tell you some thing, (although not always because I suck at grammar, too.)
9) Pictures - Often time, you can tell whether an item is real or fake by pictures. Many replica sellers don't put a real picture of the item they are selling, especially shoes and clothes. Ask them for a picture as you may find some flaws.
After all, I encourage you to buy authentic items, they will definitely worth the price you pay.
I hope you will find this guide helpful, if so, please rate this. Thank you!