This image is a still from an early 1970s Color Climax 8mm porn loop called Slave Market. (CC Film No. 1403 Slave Market, 100M/340FT. Color De Luxe) The movie was advertised on the back of the film box thusly:

‘Many strange things happen in Copenhagen’s porno underworld. If you’ve got the dough, there is no limit to your experiences: You can even take part in a sale of slave-girls and sex servants. For a couple of hundred you can buy girls in a somewhat unusual brothel and be sure they will obey you absolutely. Just like cattle the girls are paraded on the stage, tied up with strings and secured with leather collars, ready for the auction…”

A gallery here contains a few more stills and shows the box cover from the videotape era, when the movie was marketed as Slave Market: Color Climax Video 294. About 20 thumbnails of scenes within the movie can be seen here. A forum thread here offers several more larger-sized stills of the slave auction scene.
Image provenance by Bacchus at Erosblog.