I checked your listings: It’s mobile-friendly and (as you can also see in the iphone preview of the eBay window in your screenshot) it looks super fine on mobiles.
My guess is that eBay’s internal mobile checker doesn’t browse through the whole source code and so doesn’t find the code that makes the images “mobile-friendly”.
Just an idea:
Insert the following code into the “footer” field in the GarageSale preferences > General and invoke the revise command, does this make any differences?:
<style>img {max-width: 100%;}</style>
You had the same problem last year:
Regards, Kristian