Sims was tendered an offer that included a fourth-round pick as compensation for the Seahawks if he signs an offer sheet with another team that is unmatched by Seattle. What that means for the Lions is that if they simply sign him to an offer sheet, they would have to give up a fourth-round pick to the Seahawks to make the deal official.
Trading for Sims may actually be more likely, though. La Canfora explicitly said that the Lions "are strongly considering making a trade" for Sims, which would allow the team to negotiate different compensation for the offensive guard. The thing is, La Canfora also mentioned that other teams are interested in Sims, including the Bears. In that regard it seems like Seattle would have some leverage if the Lions came along and didn't want to give up as much as a fourth-rounder. At the same time, the Lions may not want to wait around for Sims to sign an offer sheet because other teams are showing interest. Perhaps a trade could be worked out to ensure Sims becomes a Lion and the compensation would be no different than if he was signed to an unmatched offer sheet.
Sims has played in the league for four seasons and has started every game he's played in since 2007. In 2006, he appeared in 14 games and made 3 starts. A year later, however, he started in all 16 games. A torn pectoral muscle ended his 2008 season after only a single game, but he returned from the injury in 2009 and started all 14 games he appeared in. It appears an ankle injury prevented him from playing in all 16 games.