You can't - vehicles are not covered by any buyer protection.
When you say that you've 'rejected the goods' does this mean that you walked away after viewing or that you went ahead and paid?
All vehicles should be examined and the paperwork checked before handing over payment, it's no different to buying one from the small ads.
If you feel that you've been misled you could post about it on the National Consumer Service forum, where you can get some good and free legal advice.