I have created a video* showing how to use my SimpleBake add-on for Blender 2.8 to easily export models and textures and upload them to Sketchfab.
SimpleBake is not specifically a Sketchfab exporter, but easier exporting to Sketchfab was my primary motivation in creating it. Blender 2.8 (and previous) provides a powerful way to create materials, but baking those materials out for use in other programs and services that use a PBR workflow (Sketchfab being one example) is time consuming and difficult - at least in my opinion.
SimpleBake aims to be a one-click-solution for baking PBR textures from Blender materials. It also bakes Colour ID maps as well as the "traditional" Cycles baking modes. See the video and also the Blender Market FAQs for some good detail on what it does and whether it might be useful to you.
*My first ever video of any kind, actually. How do regular Blender tutorial makers listen to their own voice and not cringe...
Example on Sketchfab: