In regards to trading and free agents, also pay attention to their potential rating. If you're trying to decide between cutting two players who are about the same overall rating(say 65 or so) and one has a potential rating of C and the other one A, keep the A potential player. The higher a player's potential rating, the more chance they have to shoot up in their overall rating. As can go as high as 99, Bs can go somewhere up in the mid to high 80s, and so on.
And yeah, it's really had to trade for anyone who is a franchise player at a 97 or above overall rating(guys like Tom Brady, Manning, Chris Johnson, etc.) It's possible though if you have a comparable player to trade, and either two first round draft picks or another high rating player at a position they need.