I am more inclined to believe that the buyer needed to replace a lid..

The question can only be answered by a search of the buyers premises or keeping an eye on sales over the next period of time.

The buyer has provided no proof that the lid was not delivered.

The buyer returns the item at their cost and possibly has a full set to sell. At least by getting them to return the item they have to pay something for the replacement lid.

The seller is left with a not so valuable incomplete set and loses money as well.

I agree with the seller to stick to their guns.

(I mean why would a seller send an incomplete set? )  And if the lid happens to reappear when the rest is returned then have a word with the fraud squad in your state.