Sorry this happened to you. As a seller it has happened to me too and I had to go through the trouble of receiving the returned item only to fix it very quickly just by putting a spring back in place.

First thing to check...how well does the motor turn over when you move the wheels gently with your fingers? If the gears are caught then this can cause the motor to spark and smoke because it's trying to move but can't. Otherwise you'll want to check soldering areas and motor brushes....any wires touching the chassis frame when they shouldn't be?

Best regards,

Paul

The set that you sold as previously discussed on this forum was a nice little collection but with only one item which is considered collectable and that is the SK800. Unfortunately for all of the collectors out there this particular model appears to suffer from zinz pest more than others. Its a buyer beware that the chances of the castings being in one piece after posting is a real risk and can render the item considerably less valuable. If a rough value of your version is worth about $500 and now the body and chasis have parted that means somewhere a cast part has basically fallen of then the only way to fix is to bog up or get a replacement part either new or original. These parts do not come cheap. Perhaps the new owner is somehow blaming you as the sender and asking for some money back. That's up to you. If it need new parts then Ritter Restorer in Germany would have everything that is needed or ebay. If you could ask the new owner to send some photos of the broken bits this forum will tell you what happened and what needs to be done to fix.
All the best
Roger