1) Buy from a reputable seller: Hopefully your seller has a great track record with a top seller rating. If your seller has 95-100% postiive feedback rating over many transactions, they most likely are selling authentic merchandise.
2) Be wary if the seller has multiple quantities of the exact same designer item. I always look at the sellers other items before I purchase to see what else they are selling.
3) I only buy things on ebay that I have tried on in a store first. Most sellers do not take returns, and if they do, it's not very cost effective to pay shipping both ways to return an item.
4) If the deal is too good to be true, please be cautious. Chanel handbags to not sell for under $1000,
5) Read the entire listing before purchasing. Most ebay sellers are getting their merchandise at outlets and discounters. The merchandise could have been used and returned, a display or with a small flaw. There may be a line through the label or another marking so that the item cannot be returned for full price. If the item is for your personal use this shouldn't be a problem since you got a great deal. If you are buying as a gift, it's good to know up front so you aren't disappointed.
6) Make sure the actual item that you will receive is shown in the listing. Especially with designer items, photos from all angles, tags or serial numbers and any authenticity cards or dustbags should be shown.
7) Relax. Ebay offers excellent buyer protection. If the item you received is not as described, you can contact the seller or ebay to initiate a return even if they have a no return policy. Believe me, the last thing we sellers want is an unhappy customer making negative feedback comments. Most sellers are very willing to work with you, whether or not it is their mistake.