ebay Chanels are extremely dicey, possibly even tied with Creeds off ebay. This has to do with how legendarily draconian Chanel is about inventory control. It is very rare for Chanel fragrances to fall into grey-market hands, and this lack of supply combined with the high-prices Chanels command makes them an attractive product for counterfeiters.
That said, I do find it strange that someone would bother to fake a Tester bottle, as it is more effort to engrave the marking into the bottle, and a tester will almost by definitition sell for a lower price. It would have to be a mighty crafty crook to know that:
A. Ebay is full of fake Chanels,
B. Testers are less likely to be faked, and thus more likely to be considered real by bidders, so the lower price it sells for is offset by inspiring a false confidence and getting more people to bid.
I suspect that it is probably real, but old stock, and may have "gone-off" a bit, as Chanels are notoriously wont to do (another reason to be careful with Chanels on ebay).
Unfortunately, Chanel is one of the houses that it is usually best to just buck up and pay the extra at an established store, for reasons of peace-of-mind.
Welcome to basenotes by the way