Good decision!

Regarding the seller I have no clue. Power seller not always guarantee.
So having looked at the page for N900 be prepared to get a chinese refurbished one (even this is an american seller, he may/can/will distribute chinese refurbishs).
Take a look at the headphone first, as it needs to be WH-205. And I swear it is not.
Second check for chinese stickers and a plastic protector on the back of display (with open keyboard).
Third would be to boot up and check for any old user data.

Nevertheless if any of above matches, it still may be a proper functional device. And serve you long time...
So last step check call reception/sending, loudspeaker, GPS and any other technical stuff you regularly use.