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If a vehicle is warranted and registered can it be classed as roadworthy for a selling advert?

That's a very open-ended question. Obviously, if a car is warranted and registered but has a blown up engine or has suffered major accident damage, it's not roadworthy. Also, if the warrant was issued several months ago and the vehicle now has 4 bald tyres or other safety defects, it's not roadworthy either.
However, I suspect there's a specific reason why you're asking this question (a dispute maybe?) so if you want to be more specific, we may be able to advise further.

Yes there is a dispute We sold a vehicle on Trade me stating it was roadworthy. Not as is. Vehicle was registered and warranted. Buyer never inspected vehicle or asked for it to be inspected. He purchased the vehicle then vehicle was warranted that week, required some work which was all completed, then passed its warrant. Guy picked it up in Nelson on a Friday night in May and drove it to Picton to ferry then to Wellington. He never inspected the vehicle at all in Nelson. Following week he decided to take it for another warrant which his garage failed it. He is now taking us to court because we didn't advertise it 'as is'. That after he has had NZTA do a check on garage that did our warrant, which they were cleared for.
So to sum up vehicle had new warrant the week purchased.

OK, it would appear that you have done nothing wrong. You advertised the vehicle in good faith that it was roadworthy and as it was in running/driving condition, passed a WOF and had registration, it was "roadworthy".
When advertising a car for sale it is important that it is not misrepresented (ie; had you said something like "it will easily fly through a WOF inspection", and then it failed one soon after, that could be considered misrepresentation). The same applies to the Q&A section on Trademe; it's important that the condition isn't overstated.
In reality, in your case, if the buyer has a gripe with anyone, it should be the garage that issued the WOF, not you. However, as NZTA looked into the matter and found the garage had no case to answer, it would appear that he has no grounds to complain at all to anyone.