At the bottom of the website are the customer service numbers. Start there.

If the serial number was issued by Titleist, that isn’t a guarantee. That could have been duplicated. Can you check back with the seller on the chain of custody for the club? Small resellers on the internet tend not to be able to identify counterfeit items and are hard pressed to not pass up a “good deal”. Large used/new resellers like 3Balls or 2nd Swing will take responsibility if one gets through.

I won’t buy/sell golf equipment on eBay again. If anything can go wrong, it will on eBay.

Click on the Register Clubs at the bottom of the page, and input you club type and serial number there.

But to Don's point, the counterfeit models are getting so good that it is hard to tell the difference and easy for them to take a legitimate serial number from a real club and copy it numerous times. It's even hard to do a side by side comparison for flaws. Best advice like Don said, purchase from a reliable source, a legitimate website or a person you can trust received it through Titleist or a authorized dealer.