Amazon is normally pretty good when it comes to support if the item isn't up to your expectations.
Ebay, on the other hand, can be a bit of a minefield. The majority of the "NEW" N900s are shipped from China and a most certainly refurbs. You have to be careful of some of the single unit sellers on ebay too.
A friend of mine bought an N900 from ebay that was supposed to be in "excellent condition", only for me to notice that it had been repaired badly, the screen or digitizer had been replaced (indicated by it being a different colour to mine) and because whoever repaired it had superglued the front bezel back on after stripping the screws. The device also began to suffer from the "No Telephony Functions" after a few days indicating the broken SIM contacts problem.