I had a look at your listing.
It seems you’re using a very old version of GarageSale 6 (version 6.8 to be exactly). The listings designs of this old version are not optimized for smartphones. That’s why the test says “Mobile not-compatible”.
If you want to make sure your listings are optimized for smartphones you could upgrade to GarageSale 7 where all listings design have been optimized. (You need to download the latest version of your paid design from GarageSale’s Listing design window.)