As a general rule, running backs are worth more than the other positions. Usually if you want a good running back you have to give up an even better WR or QB to get him.
Your first league is interesting because your opponent is probably weak at WR with all those RBs. You might be able to trade Marshall for a quality running back here, I could see a trade like Marshall for Benson working out for both teams, however I would be inclined to stand pat with the team you have, as you are not exactly deep at WR either. I'm usually looking to trade players only when I am deep enough at a position that I have good players on the bench every week.
The league where you have Flacco and Brees is more interesting because you have a really good player on the bench every week. You might get better offers for Brees than Flacco, and I actually don't think Brees is that much better this year, so that is something you might consider.
As to the other options:
-Do not under any circumstances trade away a valuable player for Ryan Williams. The Cardinals offensive line is putrid, and therefore so is their running offense. They are one of the worst teams in the league in terms of rushing yards.
-Sproles for Bowe sounds like a fair trade, especially if your league is PPR, and even more so if you get credit for return yardage. If you are hard up for a running back I could see making this trade. I would hesitate though, only because Bowe is one of the most targeted receivers in the league, and I would really hate to give him up. I would make the trade only if you get PPR and return yardage points and are desperate for RB help, otherwise I would have to stick with Bowe who is a top 10 WR at this point.
QB: R Griffin III, A Luck
RB: A Foster, F Gore, T Richardson, K Hunter, M Hardesty, A Green
WR: V Cruz, M Austin, P Garcon, S Moss, B Hartline
TE: J Witten
K: Matt Bryant