I don't know if they are selling images, but they allow hotlinking of images. You can do it the "long way" with their embed code, but that's for unsophisticated webmasters.
Just hotlink. You can even hotlink to a full-size, high-rez image of up to 2000 x 2000 pixels (when available) when you remove the last two characters of the image URL. Even after a DMCA take-down, Pinterest keeps it on their servers.
If someone complains to your host, your host won't do anything because the image isn't stored on their server, but on Pinterest's servers.