Is a person considered guilty and a thief merely for helping to transport stolen goods without being the original thief? And what should he do with stolen goods he took from someone who refused to return them?

Theft is one of the major sins, and aiding a thief is forbidden. One must repent to Allah for aiding a thief and for condoning evil. If you were able to recover the stolen item or inform its owner and did not do so, then you are liable for the stolen property, and it is obligatory for you to return the stolen item to its owner if you know them; otherwise, you should give it away as charity. It is not permissible to buy or accept stolen goods except for the purpose of returning them to their owner.