I'm a big Ebayer, but I wouldn't buy flea control products off Ebay. There are a lot of counterfeit products on the market. The only safe place to buy them is from your vet or a licensed online pet pharmacy like Dr. Fosters and Smith, KV Vet Supply, etc.
Flea products at the retail level are the ones that can be counterfeit. A gal on another forum got counterfeit Frontline from Petedge.
Here's an article from the EPA about the counterfeits and what to look for:
http://www.epa.gov/pesticides/factsheets/petproduct.htm