"timing belt 9 months 6200 miles sensor 7 months"
O darnn. That would be considered new in the life of Mig Ownership.
Did the belt snap then ? Sounds just like bloody bad luck. Don't think either you or the seller could have pre-emted that happening. Suspect that will be his defence, upon which you'll struggle like mad to have a sucessfull claim against him.
"New" does not guarantee brand spanking out of the box never been used new. Even if his ad HAD stated "timing belt 9 months 6200 miles sensor 7 months", I would suggest any of us buyers would have gone ahead with the purchase, its a job not needing doing for another 33,000 miles after all. With repect you too would have continued with the sale knowing so also.
IF however, it hadn't been done at all, you'd stand a good chance but if he can proof that it had been done, even 6200 miles ago, then I think you'd struggle. That said, if he can tell you who fitted it, you may have a claim through THEM, against the supplier of the kit unless the pulleys and tensioners weren't changed, in which case you MAY be able to claim against the fitters, as they should have known to change the complete kit. It's standard practice !