Ahh. I see. That's a tough nut. I'd advise giving him a few days leeway anyway - with a private seller you don't know what emergency circumstances may cause a day or 2 delay, the last thing you want to do send the police around his house if he's spent the past few days in hospital due to a car wreck. Have faith in the fact the the vast majority of people are honest and well-intentioned and try not to worry too much. With that in mind, you should set a reasonable deadline whereby you must have either the unit or tracking number, and act on that date if need be.

It's really not worth worrying until that date - until you can convince the police that you have probable cause for concern, they're not likely to act anyway, so you are effectively powerless in the situation at the moment.

Accept that and try to relax for now. If it helps, prepare for what you will do when it is time to take action - print out any correspondance, copy any transaction reciepts, investigate options for recovering your money etc.

I'll keep my fingers crossed for you.