Since you are using PayPal and Google's secure checkout sites, you could obtain badges from those services. For instance, PayPal provides a seal Verified seller accounts:
And Google provides a badge of their own:
Since your own site pages are not secure-encrypted -- nor would they have any need to be, since all carts are anonymous without any sensitive data being transmitted before checkout -- it would be inappropriate to display any security seals on your site, as those are typically only shown on pages and domains that are secured.
Once the buyer proceeds to checkout, which is the first place they would be asked to provide any personal/financial information, then they would see all the standard indications of a secure site and connection in their browser, such as the padlock icon and a page URL starting with "https://", as well as whatever additional seals and badges the payment processor may display within their checkout pages.