Your domains are not actually "listed" they are "promoted" meaning....... If someone looking to buy one of your domains and actually types it in to their partner sites, it will forward right over to afternic so they can make an offer. Your names do not show up on their partner sites under their domains for sale. Someone actually has to type it in directly. So is it worth the $19.95? Depends on what your names are imho. If they get regular type in traffic or you think someone may actually go to their partner sites and search for your exact domain, then yes it's worth it. If not, then no.
Partner sites are on afternics site, register.com buydomains.com enom.com and 2 or 3 others.