You could offer different lengths of time... and when a listing expires, it could be saved in a "Drafts" type section only the seller could view... of course with a "relist" option.
Something like:
STANDARD LISTING - Your listing automatically expires in 45 days.
EXTENDED LISTING - Your listing automatically expires in 180 days.
But I WOULD require an expiration ... as you will NEVER EVER EVER get people to "always" come in and de-list their ads when they sell.
All listing are saved in a draft for people. I'm working on an idea for relist right now. I am thinking some kind of auto relist after 30 days, then maybe after 180 days if there is no way I can detect if that person is active there items will be delisted and put into draft for them to relist when they come back. Something like that. I will send them an email telling them there item has been relisted and if its been sold click a link that should be easy. If still forsale do nothing.
I just changed the home page to show the last 20 brand new items listed on the site. It was just random stuff.