What happens if I buy a keyboard on Classifieds and the seller does not deliver the product?
First of all, you should contact the seller and ask what is up. Be polite.
Only if there is a conflict or the seller does not answer in due time should you file a complaint.
Several times I have had sellers delaying the shipment of an item for various reasons: everything from laziness to being thrown out by his girlfriend to moving house and not remembering which crate my item was packed in.
Sometimes a package can take a long time in shipping, depending on where you are in the world and if it is over a holiday.
You should have agreed on a shipping method with the seller before you paid.
If you got insured shipping, you should get a tracking number so you'll know where your package is. However, if the item gets lost or damaged in insured transit, the seller is the only person who can and should file the claim with the shipping company.
If you did not request insurance then the risk is on you: and if the package then gets lost, then all you can do is suck it up.