Things to do before buying a theme:

  • Check the demo’s performance on GTmetrix !important… From my experience, if you get an F score it is likely that you won’t be able to optimize the site much
  • Sometimes author will add admin panel screenshots, make sure to ask for the actual access to be able to check every aspect of it.
  • Check the Online Documentation (If Exists)
  • Supported Plugins: before buying a theme you should make sure it supports the plugins you will use
  • Make sure theme updates are done on a regular bases (at least once every 2 months for older themes)
  • Check the responsiveness of the theme demos yourself
  • Check browser compatibility yourself
  • Read comments (themeforest or theme support forum) !important…
  • Analyze the Quality of Support by making sure the author is patient and answer questions in a timely manner,
  • Make sure the theme is SEO friendly

If you still not sure you can ask the author some presale questions until you satisfied