There is a significant difference between the value of an original and a reprint. Without the covers, it is nearly impossible to tell by pictures which of the two you have. Do you know for sure which you have?
I have observed the loose 1964 reprints being stripped of their covers and bound as if original, therefore the validation of this information is critical to the discussion.
I would be thrilled to hear you have "originals" through the 1907 (Volume 18) bound volumes.
(The 1964 NGS reprints were only for the years 1888 through 1907)
I can determine, however, if the most valuable bound volume you have is original by positively answering two questions for me:
1) Do you have a Volume I Title page?
Discussion: The Title page was ONLY printed in the back of the Volume II, No. 5 issue in the ORIGINAL. No reprints, bound or loose, have the Volume I Title page - period! It was, in the original, located behind the combined Volume I & II Index plus a few blank pages - all by itself. If you have it, it would probably still be in that lonely location in the back of the last issue of your combined Volume I & II. However, it appears the Volume II Title page and combined Contents listing have been placed in the front of the bound volume. It could be possible the Volume I Title page has been placed with these pages, most likely at the back of the Contents listing.
2) If you do have the Volume I Title page, what is the date of the Volume printed smack dab in the center of the page?
For your information, on the only sale I have observed of a complete set of original volumes 1 through 18, please direct your attention to the bound volume section in the article linked below:
The all caps word "REPRINT" was printed in small letters on the covers of every issue, bound and loose, and placed on each cover about 2" from the bottom of the cover and centered left to right.
If this word exists - then you have reprints. Without it, you probably have originals.
If you have reprints, measure the page dimensions and compare them to your first three books, Volumes 1-6.
If Volumes 1-6 measure the same, +/- a fraction of an inch, as your reprints, then you'll know for a fact your first 6 volumes are also reprints. The originals of the first six volumes were noticeably smaller in dimensions. The reprints were purposely reproduced to be the same size as the normal NGM.
Please read the entire article I linked above. I've spent a lot of time collecting/collating data from eBay to assist anyone wishing to sell NGM's on eBay. This includes reprints (bound or loose).