Our Cart and Buy Now button URLs should not normally produce any security warnings, and just to be sure, I retested some standard Cart and Buy Now links and checked the HTML source of our cart window to confirm there is no content being loaded insecurely via normal HTTP in our cart.
If you could provide us with the "direct buy link" that you're sending to prospective buyers, we can test that link in particular or recommend a better way of sending purchase links. Feel free to post that link in reply here, or just send it in an email to Support:
https://www.e-junkie.com/ej/contact.php