Were the trades even? Any trade that is same-tier 1:1 should be easily reversible so it wouldn't matter if someone takes it.
Any time you want/need to give resources to smaller struggling players the only effective way to do it is to have them place requests because if you post offers anyone can take it.
As usual, the fog of 1:1 as "fair" clouds the judgment. ("reversible" here being a substitute for "fair.") The thing is, people don't trade 1:1 all the time and thus, "same-tier 1:1" isn't a viable answer. After all, the only thing this OP said was that the trades that were being taken and re-posted were NOT being posted to the advantage of the fellowship. To me that means they could have been 1:1 same tier trades since if a good is scarce all the trades posted might be 1:1.1 rather than 1:1. Posting a 1:1 trade in a trading environment where all other trades of the same type are 1:1.1 means you don't get your trades taken....or you get them taken by players willing to lose the profit of the 1:1.1 trade. In my experience, people willing to do that are rare. So the issue would not have been solved by a 1:1 posting alone and the problem seems to have been that the trades were being posted in larger quantities, thus making them out of reach for smaller players in the fs. Even if the trades re-posted were of the same tier 1:1 type, it's not the type of trade, but it's size, apparently, that is the problem. The same problem would be there if the trade were cross-tier or otherwise different from the 1:1 type.
The magic formula of reversibility doesn't work here if the trades weren't same tier 1:1, no matter what the size. The reason the ratio of the trade goes away from 1:1 is because there is a perceived change in value in the good. The cause of that change can be an actual production loss caused by the game mechanics, or a psychological change in the individual's perception in response to changes in his/her desires. All very complicated and very human.
I do agree that one thing you can do to at least reduce the problem is get smaller payers to post what they want rather than always looking for it.
The only thing I think can be done is for the AM to clearly state to all the players what are the limits of trading. If size matters then it should be stated and why and how it matters with it. Get feed back, discuss it, and when somebody doesn't post trades in the manner described, ask why. Sometimes there are reasons, even if the reasons sound a bit fishy. But if we at least understand them, we can better deal with the situation.