Looks like it's a time to repeat myself, and reiterate what others have pointed out in various ways (that I can recall), again, but I can't be mad about it much, not with how sprawled out this thread has become and seeking the answers already provided here would be a punishment of such cruelty that I wouldn't wish it on my enemies (half-truth).
I take no issue with newbies posting cross-tier, occasionally accept some, too, and my complaints are aimed at those who have been playing regularly for half a year or more. When you are constantly wading through a quagmire of unfair trades, it weighs on the mental fortitude of even the most diligent, and that can make keeping an eye out for the newbie posts challenging.
The main reason (bad) cross-tier are unfair is due to all players needing all three tiers, along with increasingly large amounts of each scaling up with advancement. If a majority of players are only taking in the lower tiers and not giving any back to the economy, it puts an inequitable demand on those who are producing the lower tiered goods. Lower tier also require more room for support buildings (residences, workshops, culture), and are more labor (click) intensive to create, especially factoring in the workshops necessary to keep the factories running, and there are many event/crafted buildings that give a significant supply of third tier goods for free, while the ones offering lower tier are few and far in between (with several players having stated they find such structures useless because of their lower fare). In other words, a person is(not fully comprehending that they are?) expecting another to do the hard(er) work(less fun play) for them, instead of ramping up their own lower tier output and suffering the extra clicks/less space, which, yes, would be of less ease than putting up a ton of third tier for first and second trades.