If he is asking you to send him 10% to via Western Union or to a personal bank account then it sounds like it might be a scam. I would definitely think twice about doing that... I agree with the other posters, 500$ should be enough to show serious interest.
I almost got burned on a scam like this when trying to buy a BMW S1000rr I found on Craigslist. The scamer even went as far as to create a fake PayPal invoice that instructed me to wire the money to the personal bank account of a "3rd party" who would hold the money until the transaction was complete. I called paypal to confirm the invoice and they told me it was fake and that they would never have a customer wire money to a third party.
That being said, I do believe that PayPal does offer some type of Escrow account where they will hold the money until the transaction is complete. If not, they will at least insure it if you do things from them. All the buyer beware websites always say to never send a Western Union and never send a bank draft to a personal account.
Just my 2 cents...