To list a proudct on Amazon that isn't currently in its catalogue you need an EAN or UPC. If your products don't have it then you need to buy your own.
Amazon is a gamble. In theory they could prevent external sellers listing, as I believe they do (on the platform we're on) in books. You're also passing to them details of how many you sell at what price etc. etc.
Nevertheless you have a huge 'captive' audience as many people will only buy from Amazon because they consider it's safe. Even if Amazon at some point in the future prevent you selling on Amazon, they can't prevent you selling anywhere else.
It's a choice you have to make I guess, offsetting Amazon's fees and other 'information' gains against your possible increase in sales.
Selling on Amazon is a game of cat and mouse... I sell on Amazon as a Pro-Merchant which offers my company some great advantages (sorry it is by invite by Amazon only) but I often wonder if it is worth the hassle!
I end up competing against a supposed partner, because they can see the volume of business I do with them and the products we shift, it is not uncommon for them to try to undersell me if they see I am moving product in a certain area on their website, but they rarely have the guts to do more than stick their toe in the water, so I just wait until they are back out of the game...but as Amazon accounts for only 10% of my sales, I just keep swinging in new products...watch them play catch up, and hold tight until they lose interest again.
Very satisfying knowing they can't beat me on price 90% of the time!
Amazon is a route to market and shouldnt be your only one. If you have tied supply up/exclusivity etc, they would have to buy through you and your great bonus is volume sales with less effort (and margin).
As for pro-merchant, I dont think it is invite only! See here.