Firstly you should always review the listing first look through it or scan through it to see weather the seller of the item has given away any evidence that the item is counterfit.
Secondly you should read the sellers FEEDBACK. See what other ebayers have commented on their items.
Thirdly you should check for miss - spelt words some eBayers tend to spell some listings incorrect on purpose so that other eBay members actually know that they are fake goods. Sometimes the seller will even state that the item is "Genuine" but when you recieve it, it might not actually be Genuine.
Finally check the sellers member status if you notice they have 0 feedback and are a new member (ebay will show you weather they are a new member) try to avoid buying from them. Also check weather they are a powerseller, most powersellers are and can be trusted or they risk their status being taken from them.
REMEMBER: Check the return policy. See weather you can return within 7 days of purchase.
Most powersellers, business sellers do accept returns.
Hope you like This guide! Help keep ebay a safe trading community.