With someone or an entire country? Based on what specifically? to which extent?

300 people in a 173 million people country do fraud, flag them all?

It matters less what the ratio of bad actors to population is, and more what the ratio of bad actors to legitimate orders is. In the examples I mentioned above, they were seeing hundreds, perhaps thousands of bad orders without seeing any genuine ones. From countries with known issues of trafficking stolen goods.

The truly genuine customers can contact the company directly and deal with them directly, rather than buying through a site.

When your per-capita GDP is a few dollars a day, the odds that the order for a multi-thousand-dollar consumer electronics item is genuine gets awfully close to zero.

You call it discrimination against an entire country, I call it a sensible policy for a seller to enact. If you'd like to volunteer to police these fraudulent purchases, I'm sure they'd appreciate the help, but when you have an entire department that spends most of its time dealing with orders from two countries, orders which almost always turn out to be fraud, the most sensible way forward becomes nearly self-evident.

As others have mentioned, you seem to draw no distinction between constraints on governments, justice systems, and private policy determined by individuals and companies. I think until you stop confusing the two concepts (and the vocabulary thereof), you'll have a difficult time persuading anyone to come around to your thinking. (To say nothing of forming a coherent ethical argument for why you think this is some great injustice and what possible solutions might be.)

You are not taking in consideration that poor countries have high inequality and disproportionally rich people at the top. So no, that's not correct.

I'm taking exactly that into consideration–Nigeria has a sizable population, but very few people who can actually buy expensive consumer goods from western countries. So the odds are even more likely that orders are fraudulent. This isn't a country with a giant middle class.

Those few are actually in the order of hundreds of thousands if not millions. You are talking about a 173 million people country.

If only 1% of 173 million had a western-like middle-class+ purchasing power, that gives you? 1.73 million. A lot of people you can do business with. Like 4x that of Luxembourg.

You are also not taking into consideration that I, quite frankly, do not have the time or expertise to vet everyone who tries to buy something from me.