I don't think it is simple to set a hard deadline for when the sox are buyers/sellers. The amount of uncertainty in the market, coupled with a second wildcard spot that makes the wildcard less valuable overall, sort of trumps the ability to identify which teams are buyers and sellers. Simply put, it depends on where the sox are and whether there are any opportunities to sell high on a player that we don't need.
In terms of where the sox are, I think its safe to assume they will attempt to compete next year. In order to do so, they will have to resign Lester because there is no lefthander of his caliber on the free market next year. Therefore any trade made in mid-season will be an attempt to boost the team in 2015. A quick look at the offensive/defensive/pitching numbers makes it clear where the sox problems lie: the outfield (where the red sox are dead last in production), third base, and catcher. Since catchers are a valuable commodity, and the red sox have several players who could play at third base next year, any t
rade made at the deadline will either boost outfield depth, accumulate high-ceiling prospects (unlikely), or acquire an MLB-ready excellent outfielder that is becoming too expensive for a team that is not going to win this year (e.g. Stanton. Also, completely unrealistic and improbable).
That being said, the potential list of major league players that would net something in trade is very small. Players with expiring contracts are less valuable to teams now because teams can no longer acquire draft pick compensation for a 2-3 month rental. A player like Jake Peavy may not even be worth a C-level prospect, even if the sox eat a portion of his contract. Below are a list of players that have played well this year, I've bolded my preferences for players that have solid trade value right now:
Position players:
Xander Bogaerts (1.7 fWAR, 120 wRC+, .353 BABIP)
Dustin Pedroia (1.4 fWAR, 100 wRC+, .302 BABIP)
Mike Napoli (1.2 fWAR, 129 wRC+, 0.339 BABIP)
AJP (0.8 fWAR, 91 wRC+, 0.310 BABIP)
Brock Holt (0.8 fWAR, 126 wRC+, 0.406 BABIP)
David Ortiz (0.7 fWAR, 117 wRC+, 0.251 BABIP)
Jonny Gomes (0.1 fWAR, 98 wRC+, 0.295 BABIP)
Pitchers:
John Lackey (2.3 fWAR, 3.18 ERA, 2.97 FIP)
Jon Lester (2.3 fWAR, 3.52 ERA, 2.92 FIP)
Koji Uehara (1.2 fWAR, 0.65 ERA, 1.71 FIP)
Andrew Miller (0.8 fWAR, 2.77 ERA, 1.58 FIP)
Brandon Workman (0.7 fWAR, 2.86 ERA, 3.12 FIP)
Junichi Tazawa (0.6 fWAR, 2.28 ERA, 2.79 FIP)
Jake Peavy (0.5 fWAR, 4.76 ERA, 4.78 FIP)
Burke Badenhop (0.4 fWAR, 1.77 ERA, 3.34 FIP)
Right now, trading our position players doesn't make a lot of sense. Guys like Stephen Drew and WMB have almost no trade value, and are unlikely to garner a great deal of trade value over the next three weeks. While Bogaerts is a tradeable asset, he is also putting up an incredible rookie season: Xander is third best hitter by wRC+ among all major league shortstops. Jonny Gomes has an expiring contract and can only hit lefties, making him less valuable as a trade-chip. The most interesting trade chip the red sox have is Brock Holt. He has put up great offensive numbers over the past few weeks, these numbers seem unsustainable in the long run (see his .406 BABIP), but he can play not-terrible defense for multiple positions on the field. Because he is under team control, you may be able to deal him to a team that doesn't need him this year for some semi-decent prospect or a backup outfielder that can hit righties.
On the pitching side, the sox are dealing from a position of relative strength. I wouldn't recommend trading Lester because you are unlikely recoup much value in a 2-month rental relative to draft pick compensation, and such a trade would jeopardize any ability to compete in 2015. The bullpen guys may be valuable for a GFIN, and because relievers may remain overvalued in the market, are trade chips that might net a B-level prospect or outfield depth. Workman is another interesting trade chip, but unless you are packaging him with prospects for a sure thing (e.g. betts/workman/RLDR for Jameson Tallion), he could be more valuable to the red sox as an MLB player.
The other option is to deal prospects for an outfielder like Giancarlo Stanton or (more cheaply) Heyward/Harper. However, as P91 points out, we currently have a deep farm system that has few superstar-impact players. Apart from Henry Owens or Trey Ball, we don't have potential 1/2 starters in our system. Betts and Cecchini are our greatest position prospects right now, and because those positions are in need, I'm not sure trading either makes sense. Any package for a starting outfielder will require cheap major league talent; Bogaerts is the only player that merits such a description.
EDIT: I added spaces for clarity. I clearly wrote too much garbage