So in an anarcho-capitalistic society what do you do if ...
You are with a couple of people and one guy says he'll give you something if you do something, you do the thing and come back for his end of the contract.
He refuses and won't hold up his end what do you do?
Same thing can happen when somebody approaches you and wants to borrow something for a while like a pen and then takes it and walks away. What do you do? Take it back by force, shout at him, call your friends if you can't take him alone - you do what you can.
When trading with people you gradually build trust. Take a small village society - people traded for centuries and what you mentioned was never a problem. If someone was a known thief or liar nobody traded or shared secrets with him. Most people are honest and trustworthy.