Shelving units anchored to drywall and metal studs - Load Bearing and Safety Concerns
You could hire a structural engineer to access. 

However, you do have the 'test of time' test, they've been there and are working. However, they also appear to be lightly loaded.

Those shelves appear to be very strong and likely properly installed or they would have failed already, or show signs of failing. 

You're right be concerned about the wall studs, with shelves like this, the failure point could be the studs themselves. If the stud flange is not braced then it will fail in bending, out of the wall at the top of the shelf bracket, into the wall at the bottom. The screw threads could also pull through the metal of the stud.

If you're concerned about it, than do as you suggest, it seems that you have nothing to lose by removing them. Or it's on the seller to prove that they are ok or at least provide maximum loading.