You started the Topic 18-19 September, that was the Date of my first 707 and my ~190, since then i lease or buy every 707-320B or C i can find. So i think i'am one of the reasons you can't find them for long on the used Market (400 in Service, 40 new).
New 707 are entering the used Market nearly every Game day, if you look at the beginning of a game day you will find 707-320 for lease.