Hello ImaginaryKaos,
We understand Apple Pay doesn't display as an option at checkout on eBay's website on your MacBook Pro. We also understand Apple Pay can be used on your iPhone through the eBay app, and that other websites on your Mac allow Apple Pay.
If you aren't seeing a specific option on a specific website, you may need to contact the website administrator to determine why you're not seeing this option.
To ensure you're able to Use Wallet & Apple Pay on Mac as expected, we'd recommend making sure you're using the latest version of macOS Big Sur.
You should see Apple Pay when you check out, if it's available through the website you're accessing. Please see how to Pay with Apple Pay in Safari on Mac for additional details.
If an update is needed for your Mac, please be sure to create a Time Machine backup prior to updating. These links can help:
You can also review eBay's Payment methods policy for additional details on what methods are accepted. If you still don't see Apple Pay at checkout, but it is available on other websites, we'd recommend contacting eBay. Please see eBay Customer Service.
Cheers!