I have learned this through personal experience.
There are many companies out there that have a policy that they will only honor warranties when the item is purchased from an "Authorized Dealer".
Yamaha and Dell (and many others) have this policy.
I have spoken to the customer service people of both of these companies and they assure me this is their policy.
Over 99% of the sellers on eBay and other online stores are NOT "Authorized Dealers" and you may end up with a broken computer or stereo with NO warranty. Hundreds of dollars down the drain. Most companies make it very easy to see a list of their "authorized dealers".
Also watch out for "B" stock items (especially Yamaha) that the sellers claim are "New".
Go here:
FAQ
to see Yamahas' definition of "A" and "B" stock.
"B" stock is "used/refurbished" and NOT new.
Please do your research before forking out hundreds of dollars.