My advice to you is don't wait any longer.

1. Pester Paypal because they might dismiss you due to their 30 day rule.
2. Check the original ad (on ebay) and make sure that he mentions the word: "Nokia N900". "Genuine Nokia N900" is better. Point this out to them. If Nokia isn't mentioned at all in the ad, hope that paypal don't realise this. They might dismiss you for that too.
3. The best deal you can get is getting a refund, but they will probably make you pay for return postage..the bastards. You can contest this, but if it's not too much, I wouldn't argue with the decision. Losing 20 USD is better than losing 650 USD.
4. Is there anyone near you that owns or sells a real N900? Try to take side-by-side photos. Link also to the original User Manual. This page shows some screenshots from a trusted source: http://europe.nokia.com/support/prod...ersonalisation

Good luck. Next time never buy from China sellers. No offence to Chinese guys, but you do tend to sell fakes. I got a fake 32GB pen drive from there . They wanted me to return it as a registered packet. I decided to keep it. Not worth the return postage.