Riddle me this #2

RIDDLE ME THIS

I'm built slowly, brick by brick, but one careless crack can bring me down.

Never sold in stores, yet I'm prized more than gold.

Deals, vows, and handshakes all rely on me to stand.

Once lost, I may take ages to rebuild, though nothing is guaranteed.

I'm what you place in a friend, and what a con man steals.

What am I?