I got recently a copy of the first edition of DragonQuest. It was in a ziplock bag containing all the first edition box contents plus a copy of the Palace of Ontocle and a copy of the Game Master Screen. My questions are:

1) Were the first edition of DragonQuest sold in a ziplock bag? Perhaps when SPI ran out of boxes? The ziplock bag is surely from the time...

2) How much could a DragonQuest screen being worth? I bought the lot to complete my first edition boxed set and I plan to sell everything else...

I don't think the 1st Edition was sold in ziplock either - only in a box.  But the boxes are somewhat fragile so they can easily be destroyed.  I don't remember the screen going for more that $10.  Ziplock bags from the time are easy to find.  Crazy Egor seems to have a huge supply of them - he bags just about everything in them - this is a normal practice for a number of resellers.  Titan games also seemed to have an endless supply of old ziplocks and NK has newer ones.

The boxed set is a bit hit and miss.  A lot of them sell cheap (under $10) but some others sell well.  Books by themselves probably won't get you much.