How does Amazon sell a 300.00 cartridge for 180.00?

Bill, there is no such thing as grey market, the market is always black. And you know it better than I do - New York, New York..
That piece of junk cartridge costs $5 at most, Amazon Japan price is very high.

For the first time I am agreeing with millercarbon. Go figure.

inna

... there is no such thing as grey market, the market is always black ...

This is simply mistaken. Black market merchandise is illegal, by definition. "Grey market" is merchandise outside of officially designated distribution channels. It's perfectly legal, even if it's dubious ethically.

in the old days cartridges were among the highest markup items around, about 10x their cost. Kind of like tooth brushes at the pharmacy. Insane markups.

More true for audio cables than for cartridges.

There were (and I guess to some extent still are) legal issues around so-called parallel imports or gray goods, but recent cases have changed the landscape in copyright and patent, where a first sale exhausts the right of the manufacturer to control resale outside of the "authorized" territory. I think there is still a trademark basis for materially different goods, but it’s been a while since I looked at that. (I’m retired and am not offering legal advice).
Country or region coding for DVDs is also a form of territory allocation. My recollection is that in the EU, the allocation of rights among different member countries was struck down several years ago in a case called the "pub lady" case where a woman used a decoder chip from outside the UK to capture soccer (football) signals at her pub in the UK.
Most of the legal issues are not directly aimed at consumers, but at resellers. To the consumer, the gray market (which is a recognized term in business and law) is often a boon-- it offers the availability of cheaper goods in a market that may otherwise be restricted.
The potential downside for the consumer may be support and warranty issues- my understanding is that most manufacturers will tell you to "get lost" if you buy a gray good and then seek service, parts or support. That may be less of an issue for things like cartridges or cable. The other issue, so far I know it, is that there are some well-known instances of counterfeiting higher value products. I doubt, in the case of a lower cost cartridge, that there is much incentive to do that. But, there have certainly been reports of fake fancy audio cables (having nothing to do with Amazon as far as I know) that aren’t limited to in terrorem assertions by manufacturers.
Nor am I suggesting that Amazon in this case is selling a counterfeit or gray market good. But, I know from my own experience shopping on Amazon that LP records that are considered "unofficial" sometimes show up there and believe that some of the cheaper electronics offerings are coming from resellers (third parties who sell through Amazon) who may be distributing outside of their authorized territory. I suspect that the main issue for them is losing the franchise if their contract with the manufacturer limits transnational sales outside of the authorized territory.
I’m not making any ethical judgments about any of this. Nor am I offering any legal opinions or advice. As I also noted, Amazon also benefits from economies of scale and market leverage. Look at the book market. It has changed pretty dramatically in the last decade due in large part to Amazon’s market clout. That may be a good thing for consumers. It is not so good for the "local independent book store." But, we’ve seen the same thing happen simply as a change in markets as a result of the Internet, e.g., the decline of the "brick and mortar" hi fi stores whose overhead is higher than an online retailer, having nothing to do with legal or ethical issues. It’s simply a change in how people buy products.