downloading photos to Facebook marketplace

Try restarting your mac in safe mode and then trying.

Then restart normally, and try again.

Safe mode will clear temporary files and caches, and sometimes fixes odd glitches such as this.

Start up your Mac in safe mode – Apple Support (UK)

Workaround is to first export your photos to a folder, and upload them to facebook from the folder. (Select then file>export)

In the meantime - please check you are using the file picker under the media section as shown in the screenshot.

What did you complete?

Export to a folder? What happened?

Photo picker? What did you see.

Remember we can't see your mac. No-one can help you unless you explain in detail exactly what you are doing and what is happening.

How are you trying?