Buying an iPod on eBay

As long as you are the first person registering it with Apple, I don't think so.

All you need is the serial # on the back of the unit: http://www.apple.com/register

I think so. You can read the iPod US warranty info here.

I have returned earphones to Apple under warranty, and I had to just give them the iPod's serial number. They then used the serial number to determine whether my iPod was still in Warranty or not (based on the date of purchase I gave when I registered it). I never had to show a receipt of any kind.

Things might be different over there though, so you might want to check directly with Apple.

If it has a registration date already in the system, you will need a valid purchase receipt to get the date changed.
Sometimes the dates are estimated on when they were sold by the stores.

I am planning to buy a factory sealed box.
And you are sure of this because?
Caveat emptor

If you really feel you must, check the sellers history & feedback.
Do it thru Paypal (no debit or credit card transactions)
Get something directly from the seller (not the web page) stating it is new in factory sealed box and with th eserial number (so you can check it out at http://www.apple.com/support/)

All good points form Chris. Get it in writing if you have to.

if the ebay seller bought it on jan 1 and i buy it on july 1, can i register the ipod as bought on july 1?
As long as it was not registered previously.
Again, I would definitely get a proof of purchase to include the date. model# and serial number on the paper.

can people register the ipod if it is in a sealed box
though? or do they have to open they box and register
the number on the bcak of the ipod.

The serial number is on the bottom of my iPod's box.

Heed all of the advice Chris has given you.

If it does have a registration date of 1 January, you will need proof of purchase.
Apple may still want a proof of purchase, even if it was not registered.