Those "transactions" are most likely buying and selling of emoji characters. Some "jyoshikosei" (high school aged girls) earn big bucks on the side selling them (50% cut for Line).
I use Line to communicate with my wife, who likes the emoji, which is really just a form of small talk. I 'like' that LINE has developed a medium that manages to engage entirely without resorting to hate (except for a few kawai hate emojis). I don't doubt they sell customer meta info on top of that. HOWEVER, as the above @Ken Y-N post pointed out, it's not safe for any personal info that cannot be expressed in emoji - so it's just the perfect tool for certain use cases.
Unfortunately for LINE, after earlier edging towards profitability (profits of ~60 million in 2016, 2017), they suddenly started losing big - deficits of about half a billion dollars in 2019 and 2020.