Amy, if you don't use a light box yet, try using only natural light and avoid the flash. Northern late morning or early afternoon seem best. You don't want it too harsh.
Even after doing this, you'll want to adjust them in Photoshop, or Gimp, or whatever program you use to make them brighter and remove any blue or yellow casts.
I don't think the ones in which the background looks tilted looks so good. Feels like they should be straight.
Also, what about trying props? I'm going to add some to my photos eventually too. Maybe some nice wood branch pieces painted white? That way you're earrings will still stand out, but there will be more interest and composition to the photo.
Everything looks pretty good, but I can see where you can make them even better.
Love your work and congratulations on the 3 sales you've already had for being such a new shop.