How do I set all "Buy Now" buttons/links to open in a new window/tab, but not the "Ad

I'm using MarketPress eCommerce and some of the items for sale will be through an external link while some will not. Those products that are sold through an external link will always show a "Buy Now" button instead of the "Add to Cart" (even if the option is set to show Add to Cart, it won't because it's an external product...I like that feature), but I don't want to lose visitors every time they click a product that's sold through Cafepress or others.

Has anyone else wanted to do this? How would you suggest the best way for me to add target="_blank" to these links? I tried adding it directly into the external link input area in the admin area of "Add New Product", but it added it to the href=" ...".

So what do you think?

Thanks