Yeah I'm finding it weird people are all overlooking this.
You have personal moral and ethical responsibility for any trading, buying or selling you involve yourself in. It is morally on you. If you sell a gun to a man intending who you know to either intending to do harm with it, or think it's quite possible they are, you are morally culpable. If you sell an animal to someone who you know has a reputation for torturing animals, you are morally culpable.
Likewise, if you trade/sell a soulcoin to a being which you know is likely to harm the soul within that coin, you are morally culpable. It is not a neutral act. It carries moral weight.
5E's alignments are descriptive, not prescriptive, but if you're selling soulcoins to monstrous infernal beings, it doesn't matter if it's legal or not, you're engaging in what is clearly a form of evil, so I would suggest LN is probably an inaccurate descriptor of the alignment of a character who does that unless this is just a complete moral blindspot for that character, like they've just never thought about it.
Very surprised any LG characters in the party haven't put a stop to this, honestly. Unless they're playing the characters as "Lawful Stupid" stereotypes, it being "legal" (in hell!) is irrelevant.