You could do this just to get keep best for the box. If the models are unpainted, then someone might buy them from you, but normally people who buy this stuff want to put it together themselves. The whole store army thing is not a bad idea. Of course, if you can find some rare stuff, like old IG Praetorians or funky Forge World stuff, those could be used to fancy up the store display cases.
Yes, because certain boxes come with extra arms, weapons, and stuff, and people are always adding or changing their army, people do not want to spend money on another Cadian Command Squad when all they need is to add a Flamer to it.
A good idea for when you open the store is to put of Flyers of a contest. Maybe once a month or something, break open a box with lots of Infantry, like Orks Boys, Gaunts, or Cadian IG, all have people paint them. Charge 3-5 bucks each, and have the winner get 10 to 20 dollars as store credit. What this really does is get modelers looking at the pieces. When I saw the plastics Orks from Black Reach, I was like, oh fuck, these are beautiful. This is a good way to have people come and then start collect different pieces for new armies, just cause they are impressed by the pieces. Plus left overs can go to the Bits Box.