You will need to install a user script manager extension to install this script.
This userscript adds "Export to ITAD" buttons to online game vendors who do not provide a way for sites like IsThereAnyDeal.com to query user game lists and wishlists remotely.
It is currently in beta and any bugs should be reported in the issue tracker.
Currently supported vendors are:
Use the screenshot sheet (right-click the screen-filling view and choose "View Image" to view at full size) if you need help finding the inserted buttons.