I think, even though it may suck, a trade is never a trade until it is done. I have come to terms with that over my time in fantasy hockey.
That being said, there are situations that seem fair, and some that seem unfair (in terms of ethics).
In your situation, if you received an offer and you accepted it within a days time, I definitely think that offer should stand and be fulfilled. When you come in with a counter, I personally feel that the initial offer is now "not available". If you were to clear it with the GM and state "I have a counter but would like to know if the initial offer still stands if you don't like my counter" and you get a positive response, then you have covered your ass. Otherwise, I think your S.O.L.
When I make offers and someone wants to take some time to think about it, I always cover my ass and say "If you want to take some time, is it cool if I retain the right to say no if I have changed my mind?" I've never had a GM say no (not that they really could or I could simply pull the offer) but I've learned that covering all my angles makes sure that there are no issues.
A trade is never done until players are actually moved. Ethics always play a role and are an important part of it, ocf course, but you will always come upon a GM who may not share your view of what is fair.
10tm Dynasty Lg - $70M CAP
G A PTS +/- PIM PPP SOG W GAA SV% SO
C: J. Carter, H Sedin, E. Staal
RW: P. Kane, Stewart
LW: Semin, Couture
D: Pietrangelo, Hamhuis, Subban, Carlson
G: Lundqvist, Price
UTIL: Plekanec, Ennis, Perron, D. Brown
D: B. Smith, Morrow
F: Brassard, Williams, Niederreiter, Little, Okposo, Whitney, Nyquist, Kesler
G: S. Mason, Ward
F - Z. Kassian, Spooner, Kuznetsov, Tatar, Teravainen, Stone, Connolly
D - Matt Dumba