Here are some general rules to follow on purchasing any designer or authentic goods on Ebay

1. Know your product - Do research on your item prior to purchasing on the internet (example: design, details, fabric, etc)
2. Know your seller - Learn about the seller (example: terms & condition, return policy, payment methods, feedback, etc)
3. Ask questions - If in doubt, always ask questions prior to bidding
4. Pictures - If the pictures aren't clear, always ask for more pictures
5. Location - Location, location, location! Learn where the item is from (example: overseas or abroad)
6. Packaging - Check the packaging of the item (example: learn about how authentic goods are actually packaged)
7. Return Policy - Look for sellers with a return policy- if you're unhappy with your purchase, you can always return it
8. Stock Photos - Don't bid until you get a real picture from the seller
9. Description -  Read everything prior to bidding
10. Payment - Find out what method of payments are accepted prior to bidding

Here are some reviews & guides written by other ebayers that I thought that were helpful:

Juicy Couture: Authentic Juicy Couture by ebay user: dawnbbp1 

Lacoste: Authentic Lacoste by ebay user: andrewt714 

Louis Vuitton: How to Spot Fake Louis Vuitton by ebay user: fashionphile 

Marc Jacobs: Spotting Fake Marc Jacob Shoes by ebay user: prox1mity 

Seven for All Mankind: How to Purchase Authentic Jeans by ebay user: closetparty