Hello Chris. A while ago I asked for your opinion on a complaint…

Hello Chris. A while ago I asked for your opinion on a complaint that had been put against us through the Disputes Tribunal. The basic facts of the case are this:
Almost a year ago now, we sold our sailing dinghy on Trade Me. The boat comes with a road trailer and the two were advertised together. The eventual buyer bought the boat and trailer using the 'buy now' option. He had asked that the trailer be newly warranted and we had agreed to this, dependent on the sale. When the purchase went ahead, we took the trailer for a warrant inspection. A few minor remedial issues (lights and safety chain) were identified and attended to and a warrant was issued. The trailer was also newly registered and the purchaser arranged for someone else to pick it up. When he received the boat and trailer he emailed to say he was very happy and all seemed fine. I contacted him again about a month later to remind him to send his address details so that the change of ownership could be finalised. He did this and the paperwork was completed by me.
Another month later, the purchaser contacted me again to say that he now thought the trailer was unroadworthy and within a day, was sending me rather accusatory emails, claiming we had somehow bullied the garage into issuing a warrant of fitness. I simply replied to say this was not the case and that I considered we had done everything agreed to in the sale of the boat. Increasing demands for money came from the purchaser,
The purchaser also approached the garage that had issued the warrant (we did not know about this). The garage owner (it seems) offered to carry out any repairs to the trailer and also offered him some money. I'm not exactly sure what was said between these two parties as there seem to be a number of conflicting stories.
Anyway, the purchaser then put a complaint with the NZTA against the issuing of the warrant of fitness and this complaint was upheld. We were not told of this until the end of the process and the NZTA has told us that it is not our business. This is an issue between them and the inspection agency (the garage). However, it seems that the new owner is not able to seek compensation from the garage without taking them to court (which might be costly), so he has instead laid a complaint with the disputes tribunal against us, seeking cancellation of the entire sale of the boat. This complaint has now been heard and the case has been adjourned to allow the complainant to gather more information.
At the hearing, all the information related to this case was supplied by me: complete transcripts of all emails and the NZTA documents as released to me under the Official Information Act. As a result of this, the decision was made that this case rested on an interpretation of the Consumer Trading Law. Because we had agreed to obtain a warrant of fitness for the trailer, we are to be held accountable for that warrant. It was acknowledged that we had done everything in good faith and are innocent of any wrong-doing, but we are nonetheless responsible for the fact that the warrant of fitness was subsequently annulled (not by us).
We have been asked to provide quotes for the remedial issues that were identified as being present at the time of issuing of the warrant.
In an attempt to comply with this, I have visited the garage where the trailer is stored (in Dunedin; we live in Christchurch) and found it in the scrap metal pile out the back. It seems that the current owner has simply left it there and has no intention of retrieving it. The owner of these premises did not want to know anything about it and really wanted it gone from his premises. Nonetheless, I took the measurements I needed from the trailer and we are preparing a quote for the repairs.
Sorry to have to come to you with this saga. Could you please advise me as to our obligations under the consumer law in this case? Can we be held to account for the actions of the garage that issued the warrant (the issue was minor according to the NZTA and no further action was required by them). Although we have done everything we could have done, it seems that we are now still required to do more. This is becoming rather difficult now, given the trailer has effectively been scrapped by the owner as well.
It seems that the whole thing hinges on an interpretation of the consumer trading act. Do you have any advice that might help us here?
Many thanks,
Margreet