Every good seller uses a tracking number when 
sending a package.  If they say it's been sent, 
email and ask them for the tracking number, so 
that you can see exactly where your cel is.

Check it out:

http://www.usps.com/shipping/epstrac.htm

Just plug the tracking number in here, and bingo, 
the info shows up.  If the package goes UPS or 
FedEx, here are those links.

http://www.fedex.com/us/tracking/

http://www.ups.com/tracking/tracking.html

If the seller refuses to give you a tracking 
number, or "can't find it", or says that it wasn't 
an option when he shipped your package... then you 
need to look into getting your money back.  >_<

If it turns out that you've been ripped off, 
you'll definitely want to leave negative feedback 
(if you bought through ebay or yahoo auctions).

If you paid through Paypal, you can also contact 
them and have them investigate.  If Paypal decides 
that you've been ripped off, they'll actually 
deduct that amount of money from the seller's 
account and send it back to you.

But first, ask for that tracking number.  
Because mail delays do happen sometimes.  I've 
been on both the sending and receiving end of 
packages that take forever to arrive.  It doesn't 
happen very often, but when it does, it's WAY 
frustrating to both people.

Way, way, way frustrating.