Well guild members all should know what a fair trade is so an unfair trade is the opposite.
I know the inno dev team are busy with many things and this might not seem a high priority but with more players who abuse what they can because they are allowed to should not be encouraged.
I'm not in a guild, and neither are many others, so that point doesn't always apply.
If guilds and their members require fair / equal trades then that's up to them and those who join that guild know the rules.
Again, a fair trade is one where both parties are happy to agree to it. No-one is forced into trades they don't want to do.
If any player requires goods from any era (and wants to trade over several eras to get them) why shouldn't they be allowed to do that?
I don't understand why anyone can be that bothered about a trade they don't want to be part of. If it's not for you, or you don't agree with it, then ignore it.