Why on earth did you buy from a seller with such a woeful feedback score?  Did you check prior to purchase.  Ebay Money Bank Guarantee is there for buyers in this sort of circumstance.  Do not deal directly with seller and do what he says.  Go straight to Ebay with problems.  Help at top/bottom of every page.  Scroll down to what you are after and ask to speak to a supervisor. If time is up on the MBG today I suggest you do it NOW.  Just curious, did seller give you a return label or did you just pay for it to be returned?

You make no mention of opening an item not as described dispute

You say the 'seller sent you a form'  does that mean they issued you with a proper return label, or just a 'form' with an address on it'

The post office staff don't come into it, they would be required to give you tracking for a parcel anyway

And, again, don't buy from sellers who have feedback in the gutter so you will avoid this issue in future

If you did not follow the correct eBay process, and instead 'do what the seller said' you are not covered

IF you did indeed follow the proper process, did you upload the tracking to the dispute?

Have you checked the funding source for the refund?

Honestly, ebay should eliminate all message/suggestions that buyers deal with a seller first.

There should be one method and one method only for making claims when things go wrong-all problems should go through ebay.

All payments now have to, so all problems should have to as well.

That way, you'd be covered.

You've (by the sounds of it) gone about things completely the wrong way.

Never pay for a return of defective items.

Okay, now you need to go open an ebay claim for item not as described. Follow it up, don't let it lapse.
Add your tracking details to show it was delivered to the seller. You may at least get a refund for the item this way.

The seller has no intention of replying or refunding you but if you open a proper ebay claim, that they cannot ignore.  or they can, but you'd win.

Wondering if OP is referring to a return authority/form (for want of a better term)  I believe you present the form and item to AP, you are issued a tracking no  and the original sender is billed for the return post.

Note I have no personal knowledge of this method, but believe that is how it works.

Has the tracking number been uploaded to the case,  is it actually a tracking number or just a receipt number