I disagree. A third baseman with Lowell's skillset is pretty valuable and they're not as easy as all that to find.
Look around the league. Sure there's a handful of superstars at 3B like Longoria, A-Rod, etc. but the depth of elite talent is actually shallower at 3B than it is at SS or even 2B right now. And most of what is out there is either young and underdeveloped (Gordon, Encarnacion) or old and declining (Glaus, Rolen, Lowell). Not a lot of all-star caliber 3B's in their prime right now, only really Aramis Ramirez and A-Rod.
For the most part if a team wanted a 3B on the trade market the most he'd likely come up with is either old and declining (Lowell/Glaus), or average with limited upside (Blalock/Teahen/Crede)
One of the reasons that if I had to list the guy who it would hurt us most to lose right now it'd probably be Youks.