In your stylesheet (/includes/templates/your_template/css/stylesheet.css), add

/* space & border between listing items */
.productListing-odd td, .productListing-even td {
text-align: left;
vertical-align: top;
padding:0.5em;
margin-bottom: 1em;
border-bottom: 3px double #aaccbb;
}

Change numbers, add, edit or delete properties as desired.