This suggests it was the address:

"On the customs form, I put the value at $500 and the description said “cardboard art.” I’m not completely sure of this, but I think Nigerians have to pay a small percentage of the value to customs, so putting a high price on the customs form hopefully cost them a little money."

That bit about the customs fee wouldn't make sense otherwise.

I don't believe the author meant that people should be suspicious of "Nigerians" in general, but rather that they should be suspicious of Nigerians when in the context of an online monetary transaction.

The sad truth is that a lot of scammers originate from Nigeria. Therefore, it makes sense to be extra cautious when engaging in financial transactions with someone who is in within Nigeria.

I highly doubt the author was implying any sort of racist sentiment.

There are definitely scams that certain ethnic groups are involved in (Indian tailor scams? Chinese tea/art scams?), but this has more to do with skill diffusion more than ethnicity; people willing to scam learn it from someone close to them, and it seems 419 scams have achieved critical mass in Nigeria so the diffusion is pretty broad now (couple with an economic situation that makes scamming somewhat desirable).

I've met plenty of Nigerians who are not scammers. Stereotypes melt away quickly when you meet real people, but the survival filters remain in place.