Hi Marylillis
Welcome to the Apple (user-to-user) Community.
Re: I suddenly cannot upload photos to eBay as a seller
Wondering which Mac model (year) you have,
and
Which Mac OS version is installed.
Seems iPhone and Mac would each work a little differently.
Am no expert in this area, yet your post surely deserves a response.
Perhaps the Apple Support articles below can help ?
Eg: Apple support have this older article online that may or may not help:
Upload photos from your Mac to a website - Apple Support
Latest info:
For MacOS Ventura: (else please choose your MacOS at the top of each article):
How to: Share photos using other apps and websites in Photos on Mac – Apple Support
Else:
How to: Control access to your photos on Mac – Apple Support
or
for very careful use : ie check security
How to: Allow apps to use your photos – Apple Support
All the best :-)