I would like to use "I did not receive" feature but it seems that does not work until after the "last due date" has passed.
The 'last due date' is the date that eBay and the postal service think the parcel will have arrived by.
It is often optimistic.
I allow 20 days for arrival from North America and 30 days for arrival from Asia. Usually I am pleasantly surprised.
If you have not received your order by the 'last due date' go ahead and open the Item Not Received dispute.
Do NOT allow the seller to promise a replacement. It will not arrive.
If the seller cannot prove delivery, you will be refunded in full. If the seller does not refund you, eBay / Paypal will and go after the seller for their money.
If you do not receive your whole order, open an Item Not As Described dispute.
The problem here is that the seller is allowed to demand the return of the sent goods before refunding.
If the seller is overseas this can cost $40 or more.
So.
Open the dispute.
Ask if photographs will be sufficient proof of the Not As Described problem.
If you buy from overseas, be ready to get less than you expect.
You have 60 days from purchase to leave feedback. The most effective feedback is calm and factual. "Parts missing. No replacement or refund offered." for example.
At the same time, you will be asked to leave Detailed Seller Ratings. A normal pleasant transaction gets 5 stars. The Lowest Rating is ONE STAR. No stars is a free pass.