No, it is not safe to buy flea control products from Ebay. It is actually illegal to sell them at the retail level now.
There are many counterfeit flea control products on the market according to the EPA.
Counterfeit Pesticide Products for Dogs and Cats | Pesticides | US EPA EPA orders retailers to stop sales of counterfeit flea and tick products for pets - April 1, 2004
The only way to guarantee you are getting the real thing is to buy from your vet or a reputable licensed online pharmacy. Entirely Pets has great prices on flea control products.
EntirelyPets Discount Pet meds Pet Supplies & Flea Medicine