First off, if someone posts a trade and then someone takes that trade, isn't that a tacit admission that it was, at least to the individuals involved, a "fair trade"?
I think Andari has made the most on point comment so far when he says that, at least for those who are members of a Guild which has a stated trade policy, those are really the only trade rules that matter. Four of the five Guilds in which I am a member are so-called "fair trade" Guilds, meaning that they require the 2:1/1:1/1:2 trade ratios except under certain circumstances (and the individual who posts these so-called "unfair trades" is generally required to state why they are doing so). I have known Guild leaders to kick people who would not follow trade rules, which is, in my opinion, entirely appropriate. If you are unwilling to follow the rules, don't join: there are plenty of Guilds on each server.
I think the best solution is one mentioned above: an "Only Non-Guild" Trade button to go along with the "Guild Only" Trade button. Assuming that the Guild has no stated explicit rules on what they allow as non-guild trade ratios, there should be no limits on said ratios.