The only problem with Run Until Sold listings is that sometimes people list here, on Run Until Sold - and then promptly disappear.
As an example, I purchased a product, on a run until sold, paid for it immediately, then waited for delivery. A week later, no communication from seller, and no item. I emailed them through the system to get a response a few days later - 'oh, sorry, we forgot we listed there, and we haven't got the item anymore'.
They gave a refund, no problem - but they had FORGOTTEN THEY HAD EVEN LISTED ITEMS HERE!
It almost happened a second time - but I noticed that the seller hadn't sold anything for a while, and emailed to ask if the item was still available - I never got a response.
As someone suggested above, there should be a compulsory log in time for people who use run until sold - and if sellers aren't logging in on a regular basis, then a standard email to remind them would suffice. No response? Items de-listed. Simple.