You're missing the point. In addition to Josh Tobias, the Red Sox got $13.5 million in salary relief - which will keep them under the luxury tax threshold. That has value. Significant value. The trade with the Phillies wasn't simply about trading player value for player value; it was about payroll management. This is a relatively unusual problem for the Boston Red Sox, and I suspect that many Sox fans may not yet realize that what happened here was a salary dump, not a trade of player value for player value.
I don't believe that the Red Sox "shopped Buchholz around for months." They exercised his 2017 option on November 3rd. As has been noted, on November 3rd, the Sox considered Buchholz to be worth $13.5 million; they were fine with his 2017 salary
So why trade him seven weeks later for essentially nothing? Why make a salary dump? Because, while they considered Buchholz to be "worth" $13.5 million in early November, by mid-December, they had a problem - a luxury tax problem.
What caused this was that on December 6th, the Sox acquired Chris Sale - which added Sale's $12 million salary to the Sox salary budget, pushing them over the luxury tax threshold. Sale's acquisition also rendered Buchholz surplus to the Sox' rotation.
So, perhaps the Sox "shopped Buchholz around," but not for months - for two weeks, at the end of which they made a deal, with one of the few clubs that had: 1) a need for a pitcher like Buchholz; and 2) the ability to just absorb Buchholz' $13.5 million salary.
Buchholz was "worth" only Josh Tobias in trade, because Buchholz at $13.5 million was more than most clubs could absorb - and because the Sox needed to get him off the payroll. Thus the salary dump.