I agree with Katwijk (et al) - having a feature that only allows fellowship members to trade between themselves means that other players, especially newer ones, are excluded from a critical part of the game and access to much needed non-boosted goods. Everybody complains about the empty city spaces but doesn't want to help new starters (who might find the game 'too tough' or slow and therefore abandon the game).
When I was looking for a fellowship, I applied to those fellowships which did NOT have any members in my immediate vicinity (and politely declined requests / invitations from close neighbours to join their fellowship). This means that my 'pool' of possbile trades is potentially bigger; consisting of the FS members AND all the cities in my neighbourhood. As I have expanded my neighbourhood, the number of trades available to me has grown a lot - currently there are sometimes 25 - 30 pages of trades with good variety including cross-tier trades.
It is only the discovered neighbours who get the two star trades (no trading fee) and the other cities in the neighbourhood pay a trading fee in a one star trade (assuming the trade was put up on a 1 : 1 basis on a same tier good). Obviously, the members of my FS get priority for trades but I do sometimes accept the small one star trades for my boosted goods (paying a trading fee) with the small players in my neighbourhood . If it helps them to stay in the game and give me a neighbour to discover in the future (and get full neighbourly help benefits/culture polishing/trades in non-boosted goods when my FS is in need), then I'm happy to do this.
I am fortunate that I am part of a great fellowship who communicate with each other. Many of the members advise when they are putting up trades in the chat and email functions so that members who are online at the time can get those trades and smaller players are encouraged to ask if they need anything. Implementing a fellowship-only trades feature will reduce the necessity of commicating between fellowship members (and shut out all other players) and it creates insular fellowships and silo thinking.
I support a style of play that is more open and cooperative - and I do not want to play a game that is all about conflict and aggression (the majority of these types of games are about conflict and aggression). As far as I can see, some of the most successful fellowships seem to be the ones that encourage communication and cooperation between members. Further, the smaller players could use the chat function to ask for trades in the goods they need - this would encourage communication.