You can't sign and trade in baseball. Teams have to wait until like, mid-June to trade someone who accepts a QO.
Evaluating Estrada's true talent is very tricky. Under FIP theory he was a ~2 WAR pitcher this year, but if you believe in his ability to manipulate contact quality and live in the "donut hole" then he might be closer to a ~4 WAR starting pitcher (bWAR 3.6, RA9-WAR 4.5).
Estrada hasn't really underperformed his FIP by a large amount in his career. This is basically the only year that he's done it. But, if you look at his pitch mix, through 2013 he was a different pitcher who threw ~20% curveballs. He's cut that in half in 2014/2015 and now he's 60/30/10 in FB/CH/CU frequency. And in 2014 he did underperform his FIP by a little bit.
Figuring out exactly what type of contract Marco Estrada is worth is very tricky. Watching him pitch all year makes you think there's a lot going on that FIP ignores, but there isn't a large body of data to support any conclusions on that front.
Either way, he's easily worth a QO at this point even if he's just a league average SP.
Buehrle I expect to retire right now, after what happened with his shoulder this year and him getting left off the playoff roster. This playoff run might seem like a good way to end his career. We'll see.