‘is’ usually refers to a single item/place/thing

‘are’ is plural

the part of the sentence…’separate items ‘ makes it plural.

The purchase isn’t being presented, the gold and silver are.

(of gold and silver) is a compound prepositional phrase.

Take out the prepositional phrase.

Purchase is the subject.

For example:

The necklace of pearls (is,are) on sale. Take out (of pearls)

The necklace is on sale.

The necklace are on sale.

Are you going to listen to the people that said the necklace

ARE on sale. If you were in Arkansas it would be ok but not here.

Purchase is NOT purchase are.

You have made one purchase for two items.