Classifieds, Directory – PayPal and un-encrypted buttons

We’ve set up both the Classifieds and Directory plugins to accept PayPal. However, I evidently have a setting wrong somewhere. When trying to go through the payment process, the PayPal button takes the user to PayPal, but they get this message –

“The seller accepts encrypted website payments only. You cannot pay the seller through un-encrypted buttons. Please contact your seller for more details.”

I’ve searched through the options and PayPal to figure this out, but can’t find what a setting that seems to fit. Any ideas?