I'm undefeated and in first place in one of my leagues, and have been able to make two trades in the last few weeks. Now, I overpaid a little bit to get the deals done, giving up 2 to get one top-tier option, so that might be why the deals have happened.
I personally don't have any problems with trading players to the top player in the league. There is a 6-0 team that has Brady and Cam. I was more than willing to trade him McFadden to get Cam (not straight up, but those were the principles). I worry about making my own team as good as possible, as there are very few "unbeatable" teams in fantasy. I saw a team (which I drafted, but did not own), go 15-0 in the regular season, and lose in the championship game to a team that started Pierre Thomas and Dominic Rhodes.
That being said, you will definitely face some hesitation from other owners. As said above, you just have to make them think they're getting the better end of the deal, or overpay. If you are in first, you probably have good depth, and once you are in a good position to make the playoffs, you can start planning ahead.