EZ Exporter has a starter template for the Grailed marketplace which you can customize further.

To use it, simply go to the "Templates" page and use the template called "Grailed - Published Products".

Most of the required fields are already set for you, but you'll need to modify it further to tweak the field mappings specific to your Shopify store and also to map non-standard Shopify fields such as the condition, exact_size, and shipping prices.

Our app supports exporting Metafields, so if you store those additional product/variant attributes in Shopify Metafields, you can specify them in the app to use for your mapping.