Station is epsilon eridani. Mission didn't say and doesn't say anything about illegal cargo. Source 1207 landmines from a suitable market. Convey goods to darkest high in epsilon eridani.
Useless reply
Yes, the truth is always useless. Shall I spin up a lie to make you feel better?
You were set up, lied to, cheated and robbed. It was a conspiracy set in motion by The Club, under guidance of the Thargoids to deprive you of credits, because no faction has ever requested anyone deliver any illegal goods, as everyone is a perfect, law-abiding citizen. You should launch an entire series of YouTube videos in protest, and downvote all of Reddit until they send Sandro to your home to hold your hand and tell you everything will be alright.
There, is that what you wanted to hear?
Missions will not usually tell you if cargo is illegal or not, unless they are specifically labelled as smuggling missions (Sneak in X tons of Y). But these kinds of mission happen all the time, every day, at nearly every station.
General Rule: If the Cargo could be, or is innately, harmful to anyone in any way, it's very likely illegal somewhere. If the cargo is alcoholic, narcotic, recreational, or could be used for fun, it's probably illegal somewhere, and certainly in Theocratic systems - which is also where it is most often desired.
Knowing is Half The Battle