4 Boeing 707 for $50,000. Is that possible?
Its surely possibly and there are a few things I can think of off hand, 


Missing Log Books: generally speaking if an aircraft is missing its log books you need to do quite a bit of tear down and rework on it to make it airworthy once again. In many cases the cost of this far out weighs the cost of a logbook/airworth plane so not always done. As such this can severely drive down the price of the aircraft making it close to worthless. In line with this, without the log books you can not sell any of the parts for replacement either. With out the ability to properly tag the part they are only worth their weight in scrap. Considering they seem unflyable and are fairly large to chop up and ship scrapping it can also become a costly maneuver. 
The tax man...: Airplanes (and all manner of vehicles) often change hands for strange dollar amounts due to various tax regulations. That does not look like the case here but it does happen. 
Simply Outdated: keep in mind the 707 is a fairly old plane by anyones standards these days. While it may have avionics chances are they are no newer than when it was parked there to collect dust, like it or not the best LORAN C unit in the world may have cost a fortune to install back then but is worth next to nothing now. 


These things dont exactly go up for sale all that often but here is a pretty nice one listed for 6.2 Million and here is reference to one listed for only 700K. Please keep in mind that there are countless variables for this kind of thing that could drive the price both up and down drastically.