I don't know about "only" doing apartment shops except perhaps Ellis Property [www.epmsonline.com] There are many others that do new home shops or apartments or mobile homes, but those companies generally do other stuff as well.
Remington Evaluations (remysteryshops.com) do apartment shops as well. I have completed a few for them only because they do no have many in my area. I found that when I travel outside of my city I complete more apartment shops for them. Jancyn also does apartment shops and Marketing Endeavors.
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“Lying in bed would be an altogether perfect and supreme experience if only one had a colored pencil long enough to draw on the ceiling."
~Gilbert K. Chesterton