trade resources 2-for-1 at end of round

Players could have some kind of Initiative value that is used for a tie breaker in cases like this. The turns are still simultaneous, but Initiative is used to break ties. If this case won't happen very often, then some minor way of distributing initiative would work. If this case will happen frequently (or at crucial times) then Initiative should be more than an afterthought and probably worked into the game mechanics a little. It could be something the players accumulate over time or maybe some game event forces them to trade Initiative markers (each marker having a unique number on it) with each other.

Could also have Tie tokens. The first tie is broken arbitrarily (roll a die or something) but the player who loses the tie gets a Tie token. After that, ties are broken by the player who has the most tie tokens (the loser getting a tie token each time). When players have the same number of tie tokens, the tie is broken arbitrarily again but the loser still gets a tie token so he'll win for sure next time. There's at least one game available now that does this, but its name escapes me, and there are probably several.

You could just refuse to break ties. If multiple players want the last token of a specific type, then none of them get it. If such conflicts will not occur too often, then the simple rule avoids squabbling over the last piece.

An alternative is to let players resolve one exchange at a time, going around the table. Even though the game doesn't use clockwise turn order for other things, people will still be seated around a table. If another player and I both want the last of two different resources, we'll each get one of what we want. (This makes more sense if conflicts like this happen often.)

"You could even *always* do this, as it will certainly always be to your advantage to prevent another player from getting something he needs."

OK. Another option is just to prohibit trading if there are fewer of that resource left than the number of players. Same effect, perhaps, but no bad feelings.

you go with the monopoly concept (not the game, the economic one) and use the idea that its easier to obtain the resource if you control the market. Thus the when players are in contention for the last resource cube the player that has the "most" of said resource alway has the option to take the last one first. If that player passes it moves on to player with the 2nd highest number of the said resource.