The seller can select a "bolded" status to be applied to their listing during the listing placement process. This feature can be charged for and pricing is set within each price plan. This feature can be charged differently per price plan and within fee-based price plans can be charged differently per category. Listings that have been "bolded" will have different css properties applied to them which you control within the css applied to the design. You can manage these css attributes through edits to the custom.css file of your current template set. The default "bolding" css changes the background color for the bolded listings row/gallery box and the title for bolded listings will appear in a bolder font. Those are just the default css for bolded listings. You have control of that css and so can make bolded listings appear any way you wish as long as more attention is drawn to bolded listings within your design.

If you wish to edit the css attached to bolded listings look to these within the default.css file to change:

.row_even_highlight
.row_odd_highlight
.browsing_result_table_body_even_bold
.browsing_result_table_body_odd_bold
.seller_result_table_body_even_bold 
.seller_result_table_body_odd_bold