Our FS does not allow unfair trades. Nor do we allow cross-tier fair trades. 0 and 1 star are unfair. 2 and 3 star are fair. The game has this rating system to help weed out what a trader does and does not want.
I'll put it in another way for you
Scenario 1:
John and Peter are random strangers. John's boost is tree gum and Peter's boost is platinum. Tree gum is in short supply on their server. John posts a 1-star trade - 10k Tree gum for 30k Platinum. Peter sees it and takes it because he can make more platinum but he can't get tree gum otherwise. According to you, this is an unfair deal
Scenario 2:
John and Peter are random strangers. John's boost is tree gum and Peter's boost is platinum. Tree gum is in short supply on their server. Peter has lots of Platinum but very little Tree Gum. He decides to post a 3-star trade - 30k Platinum for 10k Tree Gum. John sees it and takes it, because it is a good deal. According to you, this is a fair deal.
I hope by now you see that the scenarios are two sides of the same coin. In both scenarios, John got 30k platinum in exchange for only 10k tree gum. Either the deal was unfair towards Peter in both situations, or unfair in neither.
The tooltip provided by Inno themselves state this:
3-star: This offer is a BARGAIN
2-star: This is a FAIR offer
1-star: This offer is SOMEWHAT EXPENSIVE
0-star: This offer is VERY EXPENSIVE
They are all in relative financial terms, not moral terms. And exactly ONE scenario is stated as "FAIR" and that is 1:1