Create a gameplay report over at
https://jartexnetwork.com/forums/gameplay-reports.60/ and report the player for TP Trapping rule break.
Sending teleport request to someone, by which warping them to an unsafe area and trapping them is not allowed and is bannable.
Make sure you have sufficient evidence, as screenshots, most of times are not enough. You need to have proper recording of them inviting you and then killing you in a trap. You can also go back to the location if you remember its coordinate and recording/report the trap which they have build there for killing.
Apart from this sorry to inform you this but the stuff which you lost to them is not recoverable in most cases, and the average best outcome if they are cheating is them being banned.