FBA is an option where Amazon will store your products in their warehouses and ship the products to customers as they buy. There is a fee for this service but I find the actual cost of shipping is cheaper than Australia Post, so it’s cost effective, and you aren’t worrying about packaging and shipping products. Amazon manages all of the returns and tracking notifications as well.
If you want to sell in multiple Amazon marketplaces, i.e Australia, USA, Japan it is better to send products to warehouses in each country you want to sell in. This means you would have 50 units sent to Australia, 50 to the USA and 50 to Japan for example. To do this you need an account in each country and create a shipping plan for each country.
There is an option for Amazon to list your products for sale in other countries, but the shipping charged is very expensive and customers have to wait for the postage, so it’s not the quick Amazon delivery they expect. This can be found under settings/Fulfillment by Amazon Settings and “Allow Amazon to buy my products to sell globally”. This option doesn’t hurt to have on, but it’s not really testing a marketplace as the shipping makes it a very expensive purchase.