Amazon has pro selling accounts $40/month
it's different than doing the ebay/garage sales thing, it's designed more for corporate/volume sales
anyone can sign up with a ss but it's better if you're under a corp using ein
the key of the game is to find a source of already popular/established items, if you can undercut the the competition and take control of the 'buy box' what amazon calls the default seller of the 'Add to Cart' button you can make lots of money fast, and it's fairly easy but you have to have balls to pull a purchase in quantities/volume, you research which asin/upc numbers perform well and try to get an edge with the manufacturer/distributor price wise, you do the math and if you can squeeze enough value to make it worthwhile you go for it
there are many variables that will determine if you can/should sell an item, some categories are restricted, apparels and food items are but they can be the best sellers, food moves fast, apparels you can source really cheaply leaving you a big margin, you don't want to compete with items that amazon themselves are already selling most of the time, some times it can be a gold mine when amazon sale out an item from their inventory you take over meanwhile they reup, a day or hours can mean huge sales
they also have amazon fba fulfilled by amazon, it's a killer and a godsend, where you send your inventory to be stored, packed and shipped by amazon from their warehouse, there are storage fees packing fees etc that can eat your margins but it's good if you're like me who don't spend much time in the usa anymore or don't want to leave high value items entrusted to someone/somewhere, lots of benefits also like being prime eligible and winning the default seller slot, at the end it becomes just prepare a shipment and watch money come in if you make the right bets
eg i've had success with these little coffee things that are popular with the millennials, 5k invoiced from a regional kraft foods distributor (go pats) delivered to so fla reshipped saia freight to kentucky for around 33% roi all for cumulative only a few hours of work on every reup, many different ways to squeeze in to the amazon machine, it's fun akin to gambling in a way