I took a 500.00 deposit from someone toward the purchase of my car. On his receipt I wrote. 500 deposit toward purchase price of 7500.00 to be picked up last week of July, 1st week of August 2008. The guy hasn't returned any phone calls or emails and its well past the 1st week of August. I have passed up muliple sales because of this. Do I have to return this deposit in the event he ever contacts me? Or am I expected to hold this 500 forever and take the loss [of lost sales]?
Thank you, in advance.

I would send him a certified letter telling him that you had an agreement to pick up the car by 8/10. Since he has not shown, nor has he retruend your attempts at contact, you consider the deposit forfeit and you be putting the car back on the market in <insert date no more than 1 week in the future>.

I would deposit the money but I would hold it for a few more months in case he tries to get it back

In addition to ScottGem's good answer we would like to point out that if you end up selling the car for more than $500 less than this person agreed to pay for it you could, theoretically, sue the person for the difference.

In either scenario, you have a legal obligation to mitigate your damages.

In the here and now that means giving him notice that you will consider the deposit forfeit if he does not complete the contract.

If the deposit is forfeit you need to try to get the best price possible for the car from another buyer.