Why didn't the person sent me a check for the items I sold on Craiglist as he said he would last week?

I was selling Elvis Presley's card collection on Craiglist. Someone emailed me and contact by my phone and said he would like to buy it. He asked me for the address so he can send the money by mail before I ship it to him. I gave him the address. Now as it has been more than four days, he haven't sent any. Did he changed his mind?