Is ebay safe to purchase Precious Metals from? Well there are more fakes on ebay than from any PM dealer. So to be safe, buy from a reputable dealer.
If you have to buy from eBay: note the following.
If the price is too good to be true, it's probably a fake.
Does the seller's description of the item give you confidence that they know what they're selling.
Ask the seller questions , (get a feel of what the seller is like from their answers), about the coin to remove all doubt. Is it real? Genuine/original packaging? In perfect condition as supplied by the mint?...etc. The answers will help if you have to claim through ebay's buyer protection.
As Steve said, if the picture is poor, walk away, or ask for better pictures to be sent. If no clear pictures arrive, probably a fake.
Know how to check the coin you've bought before leaving any feedback. Weigh the coin, measure it, both thickness and diameter. Learn how to perform a specific gravity test. Perform a ping test and for silver a magnet slide test.
Put a picture of the coin up on the forum and ask for people's opinion on its authenticity.
Compare the pictures of the coin you intend to buy with pictures of a know example of a real coin.
Google what errors to look out for in a fake coin of the type you want to buy.
Don't buy from sellers who hide their feedback as private.
Only buy from sellers who have sold coins/bars in the past.
Only buy from sellers with at least 50-100 transactions with 100% positive feedback.
Very doggy to buy from China..fake capital of the world, unless you're really confident.
Use your common sense. If something feels wrong, walk away.
Thinks that's about it from me, someone will add more I'm sure.