How does Amazon sell a 300.00 cartridge for 180.00?

Yeah. A couple things.
1. Its not a $300 cartridge. Who knows what it is? Cartridge retail prices are notoriously inflated, and have been for as long as I can remember, which is about 50 years.
2. Amazon isn't Amazon, its a zillion individuals using Amazon to reach buyers, which as such its marginal income, which as such even one dollar is better than zero.
3. At Amazon the product isn't the product. The product is YOU! Like the Goolag and cell phones and so much else today, a lot of the real costs and profits are hidden. Your browsing history is worth a lot more than you think.

Amazon will also carry gray market and bootlegs until they get notified otherwise. And, I know from people who are in record business retail that in many cases, their prices are at cost for retailers- on occasion, a friend who has an online shop will tell me not to buy from him and just buy it at Amazon because he can’t resell at the price Amazon offers. They may also benefit from economies of scale in certain instances and leverage given their size and market power.
Not intended as a complete answer to your question, but a few observations about how some of their products can be priced so low.

The Denon 103 sold for $160 back in '78. And that was with a 30% markup from dealer cost! Anybody got a time machine?