This. Take it in and ask them if they would also run the serial number and make sure it checks out.
Another thing you could do is find out the weight in grams of an authentic cuff and then weigh this one. That's the first thing I did with my love bracelet. if you can get your hands on a very precise kitchen scale or jewellers scale that should do the trick!
Barring that, you could take it to a local jeweller and have them at least authenticate the metals. If it's 18K gold it MAY just be an older version.
The space between... that could be age. Honestly depending how old it is I think the font on the inscription could be as well (although it does seem too thick, not fine enough). I've seen some that I think are real but the font was different 20 years ago.
You could also call Cartier and ask them to run the number - the worst they can do is say they don't do that!
Don't worry. Ebay ALWAYS sides with the buyer. You have nothing to worry about - I have been on ebay forever and I know they always side with the buyer... buyer protection should kick in once you say it's authentic. Another good piece of advice - stay nice ... don't accuse the seller, don't make them get their back up - just say you have tons of friends who've authenticated it and that you believe it to be a fake. Paypal will help you don't worry! Send them the link to this thread!! They will ask for any supporting documentation - send them this thread.
I had a recent situation with a rose gold one. I ordered yellow gold and it was sold on ebay as yellow. I believe it was authentic and beautiful... but it was the wrong colour! Seller was lovely and took it back but I pushed for the duty and shipping too b/c it was misrepresented.