It's stating the obvious, but the point of "selling" at the trade deadline is to get value back for future free agents when you can't win immediately. Now, whether or not the Red Sox have much of a chance of making the postseason if they don't pick up anyone at the trade deadline, it's worth considering whether or not there's any real value to be had in selling should the Sox decide they're too far behind to bring in help.
The issues at hand here are draft picks and prospects. The two players who the Red Sox would be most capable of getting a return for are Adrian Beltre and Victor Martinez, and the two players who the Red Sox will have the most difficulty replacing next year are Adrian Beltre and Victor Martinez. The 2011 free agent class is all-but devoid of talent in those positions aside from those two players themselves, and given the scarcity of quality players, the trade market is similarly barren.
On the surface, this doesn't seem such a big issue. If the Sox aren't going to win, they can conceivably deal both players for prospects, and then bring them back in three months' time, picking up a few future pieces in exchange for renting out the team's stars. The wrench in the equation lies in free agency compensation and the 2011 draft.
The actual ability of the Red Sox to sign Beltre or Martinez after the year isn't much of an issue, really. Unless the trade really hurts the relationship between the players and the team, or somehow sets them up with a "perfect fit," then there's no reason the deal should make it harder for the Sox to bring them back for 2011 then if the team had held onto them. What does change, though, is that the Red Sox will almost certainly be giving up a first and second round draft pick in one of the deepest drafts in recent years to whoever they trade them to. Instead of "Beltre and Martinez for prospects," the deal becomes "Beltre, Martinez, and draft picks for prospects." Not nearly as tempting.
Even if we can't sign them back, or don't want to, the return is likely greatest if they hold on. By offering them arbitration, the Sox would receive a pair of first round draft picks and sandwich picks in return.
It all comes down to the value of the prospects the Red Sox would receive in any trade, to be certain. But given the buying power of the Red Sox in the draft, the team's trade partners would have to give up more than just a blue chip prospect in any deal. Considering that the team picked up a talent like Ranaudo (assuming he does in fact sign) this year with a sandwich pick, what could they do next year in a stronger draft?
If some team is willing to really break the bank for Beltre or Martinez, then the Sox will ultimately have to at least consider calling it a season in order to get better in the future. But the return would have to be huge -- worth a first round draft pick, half a year of an MVP candidate (at least in the case of Beltre), and the team's playoff chances (however slim they may be). Ultimately, there may be no deal out there that actually provides the Red Sox with any real incentive to sell.