If I code themeforest figma design resources, Can I sell them on my website

If I buy and code themeforest figma design resources with extended licence, Can I sell them on my website, themeforest or another marketplace? e.g figma to shopify template

You’d need permission from the original author

When I contact the original author, it is allowed, but How can I prove this in the future regarding copyright?

You would need to have:
a) an extended license
b) get the original author to provide written consent to you (for any other marketplaces)
c) get the author to submit a ticket to envato granting permission and share with you the ticket ID to include in your submissions (for envato marketplaces)

You do not get ‘copyright’ - that remains with the author unless they are willing to give it up privately, you get license to use the item.