TPA killing is a very touchy topic. I'll let you know what I've heard, and any of the staff can either back me or prove me wrong.
TPA killing is ALLOWED unless both parties agree that there will be no killing. If an agreement is made and someone goes back on it, the victim can report it and staff can punish the one who broke it if they can find the chat logs where the agreement took place. I personally recommend screenshotting the conversation to make things go smoother and quicker.
Personally, I recommend not tp'ing to someone you don't know. Or, if you need to have a trade or transaction, meet in /warp market where PVP is off, and/or have a staff member oversee the transaction to ensure it goes off smoothly. I offer myself for this as well, if I'm online when needed.