newbie, disappointed, curious..

I answered a classified for an amp that was listed for sale or trade. Contacted person and made a deal to trade amps. I suggested that we trade and make sure we are both happy and then finalize deal. He would rather finalize now so we made the deal and exchanged shipping information, etc. Today I rcv'd an email from him stating that he changed his mind- he needed the money. I assume that this is not the norm.. what do you think.. Isn't what we say (write) our word. I would not think of making a deal with someone and not following through, unless the item was stolen from my house before I could ship it.
Always learning,
zen