Depends what you're asking for it aand if he is prepared to put down any good faith money (as a non refundable deposit).
If you're asking $30k for a 2004, I would probably ask for something like that before I shelled out more than the avergae 2004 was asking. BUT, if you are asking market value ($18K to $22K), then he is well within his rights to bring his own mechanic along to inspect it or ask that it gets inspected at XXX place at his expense.
Always caveat emptor, so he's doing the right thing. Whether you indulge his request at you expense is a decision only you can make.
For me, a good service history is worth more than a 100 point inspection of the vehicles condition right now.