I'm not sure many people will bite at the trade. After all, they bought the N810 over the N800 originally.

You might have better luck selling the N800 and using the proceeds + more cash to buy your 810. I don't think you would be able to buy a used 810 for less than about $330 and they can be bought new for around $350, so it's not like you are going to loose much by buying new vs used.

Most N800 sell on E-Bay for $200+ when factoring shipping.