Hi I sculpted Melvin in Blender 2.8 and I cannot get it to upload to sketchfab no matter how much I decimate it. I contacted sketchfab for help and am waiting for a reply but I thought maybe this was a blender 2.8 issue. Can you upload 2.8 stuff to sketchfab?
I also was unable to open my 2.8 Melvin in 2.79.
I don't think Sketchfab accepts 2.8 .blend files yet. And you can't open those files in 2.79. BUT you can append the model from the 2.8 file into 2.79.