why does does my iMac not let me get Ebay on it?

FWIW, if you mean the "eBay Selling & Shopping" app, if you have an M1/M2 Mac you can get it from the App Store in the iPhone & iPad section.

https://apps.apple.com/us/app/ebay-selling-shopping-online/

some apps in app store on ios device is pretty much just a webclient version of a normal website which may be less optimal to access using the mobile webbrowser

but on normal computers people just use their webbrowser to access the site like

Electronics, Cars, Fashion, Collectibles & More | eBay

Oh,oh, it's not eBay apps for Mac, sorry, however it's a Shopping Assistant for eBay.

Apparently there isn't one for Macs.

eBay apps for Mac is available at App Store in Japan.

Have the link?

No problem