To expand on this, if you offer nothing and request all of their techs and resources, they will naturally reject the trade. However, if you then just add one of their colonies they will accept the offer. You can add all of their colonies to the list of what they offer and they will still accept the trade.
Another oddity that I mentioned in previous alpha versions that is still in 0.50 is that you can request all of their techs and resources and offer nothing, then click on the "Persuade" button, then add 10 credits to what you offer and they will accept the trade.
Somebody needs to look at these 2 issues with trading since they are MAJOR flaws.