Is Apple warranty valid if NOT buying from an authorized reseller?

Normally the warranty will be valid unless the serial number has been removed or defaced. Note, though, that if you buy from an nonauthorized vendor, the warranty starts from the date that vendor bought the system (or at whatever date the authorized dealer sold the system), not from the date you buy the system. So you will not get a full year coverage unless you buy from Apple or an authorized Apple dealer.

Warranty is tied to the computer and not the purchaser so it really depends on how the MP was procured in the first place.

If the seller got it from a wholesaler and hence, is considered to be at the retail level, then even though they are not specifically an Apple dealer it is considered a new purchase (assuming it's actually new) and you warranty starts at your time of purchase (sales receipt date).

Now, if it's 2nd hand the warranty coverage is from the time the seller purchased it and will cover you until the corresponding warranty date.

As long as the thing isn't stolen you're fine.

unless you buy a "grey market" machine, it's usually covered. The only way to truly know is to have the serial of the machine in question and call apple and ask if it's covered. Grey market means it came over here from some other country and wasn't manufactured for America.

If the seller got it from a wholesaler and hence, is considered to be at the retail level, then even though they are not specifically an Apple dealer it is considered a new purchase (assuming it's actually new) and you warranty starts at your time of purchase (sales receipt date).

Wholesalers are not supposed to sell Apple products to any non-authorized dealer, so the warranty still may start when the wholesaler sold the system to the non-authorized vendor.. The only way to be sure of getting a full warranty from the date the user purchases the system is to buy from an Apple-authorized vendor. I've seen Apple deny full coverage in exactly these circumstances.

Speak to the original owner and ask them to register it and then have it transfered to your name. They can do that by calling AppleCare.