You mentioned the relationship you established with this fellow- mayby it became too close and he was reluctant to approach you over the 600 dollars. You may have forgotten a cardinal rule of selling, "always be closing".
This customer went too long without being closed, you have to strike when the iron is hot and after one of you sessions with him you should have sealed the deal-not waited for him to close YOU. People WANT to be sold.
Now, don't become a victim over this one loss and do not become hardened to customers over it. Learn from the incident and don't forget that your mission is to sell product, you became caught up in a personal relationship you assumed you had formed and it cost you money and confidence. Sure , you were treated poorly and many of those posting want to give you a shoulder to lean on because they would never do such a thing- but you need to examine the whole scenario again and use it as a learning experience. The easy way out is to blame the customer for the loss. Good luck!