There's probably less than 50 people in the world that actually need a sidemount breather for a specific dive objective. If you add onto that the 100-200 or so that just love sidemount, you're not left with a ton of units in the market.
In both cases the only real reasons for a person to sell their unit are if they're done with rebreathers, need the money, or they achieved their dive goal with the unit. Neither group is likely to sell theirs to upgrade since the only other production grade sidemount unit that's widely available is the sf2. The die-hard sidemounters will probably never upgrade to a backmount unit either. On top of that, the explorers are likely to keep theirs since they may need it for a future project.
Not trying to be a downer, but it's definitely an uphill battle finding one used. If somebody else comes out with the next great sidemount rebreather the market will probably be flooded with sk's.
Good luck with your search though.