Trading away prospects is a good idea and approach but it depends on the team. I think as a player, I at least get carried away with wanting to win nownownow and forget my long-term plan which is to rebuild this franchise. I have to be really careful about giving away too much.
Most of the time even for a 3* SP the other GM wants an established all-star position player plus a top prospect, my first born and a kidney. I have trading difficulty on "hard" (I had it on average the first year but it was getting too easy so I roleplay this as the other GMs getting wise to my trading gimmicks) and it is definitely much more difficult now.
I covet my good players so I mostly bargain-hunt and look for "undervalued assets" that have a good chance of becoming 3* in a year or two. But that means probably putting up with another year or two of "sigh, will you PLEASE throw strikes" and "not ANOTHER home run!"
Bullpen I have never had a problem with. IME the AI grossly overvalues established closers and undervalues mid-level to setup-level arms, many of which are perfectly good enough to do closer by committee and occasionally one is good enough to close on his own. I did end up trading away my current closer (plus some filler) for that SP. It's a bit of a gamble but the closer is 27 and I have numerous 23-25 pitchers close to closer level, so I figured the SP was more important. I will miss his lights-out innings though, and I'm sure I'll lose a game or two in the ninth that I wouldn't have..