Is there more than one eBay account using that computer? You may be logged in under a different name than the one you bought with.
How long has it been since you purchased?
First Contact the seller and ask what service was used, when the item was posted and the tracking number.
If you do not get a satisfactory* answer and-
If more than 15 days have passed since you bought from Europe.
If more than 20 days have passed since you bought from North America.
If more than 30 days have passed since you bought from Asia.
Stop fussing about tracking and go to the Resolution Centre at the bottom of this page and open an Item Not Received dispute.
You will already have contacted the seller, so go to Step Two.
If the seller cannot prove delivery (the only real purpose of the tracking number) you will be refunded.
If the seller does not refund* himself, Paypal will and go after him for their money. He will have other repercussions.
*An offer of a replacement, particularly from an Asian seller, is NOT a satisfactory response.