Defaults make future listing creation easier by setting default listing values, but you can still easily editable for individual or groups of products later.
Here is how you can set your default values for Amazon.
Open marketplace connect > Settings > Update Amazon defaults
‘Web Price’ creates a live sync to your store prices (change a price in Shopify and marketplace listings get updated automatically) but you can also choose a constant value (no live sync), % or $ increase/decrease (relative to your web price with live sync) and apply rounding if desired.
'Stock Level’ creates a live sync to your Shopify inventory (which gets automatically reduced after sales on marketplaces) but you can also choose a fixed value (no live sync), a buffer (e.g. choosing a buffer of 2 when your webstore inventory is 5 will display 3 on listings) and max quantity (e.g. choosing a max quantity of 5 will always show 5 on listings unless webstore inventory drops below 5 where it will show the actual inventory).
Choose merchant fulfilled or FBA (Fulfilled by Amazon).
Merchant fulfilled using shipping rules set in Amazon Seller Central (shipping can't be set in Marketplace Connect because Amazon provides no Shipping API). From a quantity standpoint, listings use webstore quantities/Marketplace Connect settings.
FBA means listings use FBA shipping rules. From a quantity standpoint, listings ignore webstore quantities/Marketplace Connect settings and listings use quantities in Amazon fulfilment centres.