I am a dedicated Chanel fan and have purchased many Chanel products. There are many Chanel products on ebay and many of them are authentic but if you want to be sure, just remember:
Chanel products are of superior quality. If you purchase an item that is leaking, damaged, scratched etc - chances are it isn't real.
Chanel purfume bottles with crystal lids actually have glass lids - not plastic lids.
Pieces of the bottle should not break or fall off
Chanel makeup (eyeshadows etc) are embossed with the Chanel logo and look completely luxurious. There were 100s of imitation Chanel make up kits on eebs a while ago and it was clear - just by looking at them - that they were fakes. Even the Chanel logo on the front was sratched and crooked!
The fake purfumes come in realistic looking packages with plastic shrink wrap - they are good fakes. Be on the look out for poor printing or crooked emblems.
The bottom of the glass lid bottles will have Chanel written on them with other details.
Chanel products are the absolute best - just be wary of fakes. Usually if it is REALLY cheap and seems too good to be true - it is!!!