The purpose of a tracking number is to give the buyer the warm fuzzies.
It doesn't move your parcel any faster.
It doesn't insure more gentle treatment.
It doesn't even insure that it will arrive.
What helps?
If a parcel does not arrive in a reasonable time,first contact the seller and ask the date of shipping, the service used and the tracking number. (Again the tracking number is not important, but does notify the seller that you know what you are doing.)
A reasonable time in my opinion and experience would be:
15 days from Europe
20 days from North America
30 days from Asia.
An offer of a replacement is not a reasonable answer, it is a scam.
Go to the Resolution Centre at the bottom of this page. (Or to the Paypal Resolution Centre).
They will ask if you have contacted your seller, which would be the polite first step. You have so go to Step Two and escalate to a claim.
If the seller cannot prove delivery (the seller's purpose for a tracking number) he is required to refund you.
Again an offer of a replacement is not a reasonable answer, it is a scam.
If he does not send money promptly, Paypal will refund you and go after him for their money.
You have 180 days for an Item Not Received Claim on Paypal.