Should the Red Sox be buyers or sellers? The correct answer is "no." The Sox should neither buy or sell, unless presented with extremely juicy deals.
If the Red Sox were a blackjack hand, they'd be a hard 16. The trickiest hand to play. Do you hit and likely bust? Do you stay and likely get beaten by the dealer? No matter what you do, it probably won't work out. Your best hope is the dealer busts.
The Sox are only 4 games out of a Wild Card spot, but they have 4 teams to leapfrog over. They're 10th out of 14 teams in the American League. They're not quite out of it, and not quite into it.
It's conceivable that if Beckett and Lester pitch up to their capabilities, the Red Sox could go on a serious run. They could claim a Wild Card spot, win the play-in, and even win a playoff series.
But there is no move (apart from acquiring Felix Hernandez), that could spark this run. Trade deadline moves typically add missing or complimentary parts to help an already well-run machine function at peak efficiency.
The Sox are more than just a tune-up or a spare part away from playing well. They already have an abundance of bit players. What's kept them from winning has been their best two pitchers playing like their two worst pitchers.
And if the Sox do go on a run, it will be because those pitchers perform at their top level. Not because they acquire some right-handed bat at the deadline, or a lefty set-up guy.
The Sox shouldn't sell because they're not out of it. They have a chance.
They shouldn't buy either. They theoretically possess the talent to. If they do win, it will be because of the players already on the roster.