Client not paying remaining 50%

Hey guys,

Wondering your advice on this.

I have a client who has paid 50% of the amount I quoted him for his website design and coding. I have completed the project, and I have zipped the files ready to be sent to him.

However, he told me to invoice him for it and send him a request via PayPal. That was the beginning of last week. He has ignored my e-mails, and Private Messages on WebHostingTalk.com (He has been online; his profile says so).

What should I do?

Should I try and sell the design for the remaining 50%? And keep his 50%? (Maybe e-mail him saying if I don't get a payment in the next 5 days I'll be putting the design up for auction, and you won't get your deposit back so far).

I have spent a lot of time on this project for him and to be treated like this is disgusting; late nights, the amount of revisions was unbelievable.

What would you do?

Any advice or opinions would be appreciated.