Help valuing an early 1970s Marion Kops mini dress

Hi all, I've had a dress for decades now that used to be my mom's. She bought it in the early 1970s. I'm thinking of selling it. It's in very good condition. I've looked online and am having trouble finding out how much it might be worth. There is no label saying what the dress is made out of. If it's synthetic, it's very soft and silky.

Check out the photos. (Since the dress has no belt loops, I couldn't take photos with the dress hanging with the belt attached. I need to get a mannequin or take a self portrait.)

 

I'll check out Ruby Lane. I've had a hard time finding out anything about it. Most of the time you can find photos of the same dress, but not this one.
I have no idea if it was shortened. The stitching on the dress and the built-in slip is very well done. There were a lot of long-sleeved mini dresses around in those days.