Stop products from turning off - red light

[DanceVinylOnline]

Hi all, heres my problem...I am sure its been asked many times before and if someone can point me in the right direction i would be much appreciated. I run a catalogue of over 7000 items and i have them on buy now or out of stock. The situation is when a customer buys a record and its the last one the product turns off to red light so no longer able to search for it. As we have a wishlist and a CDR back up feature it is important that they stay green light. How can i solve this problem? I am a bit of a newbie so would prefer simple instructions...lol.