Kind Of Blue 1st pressing stereo...

I picked up a mint copy of this 1st pressing, the guys at the record store thought it was mono, but it is actually stereo, the labelling on the back is a bit confusing, under the columbia logo it says mono-clxxx, but in the upper right corner it says stereo-csxxx... I went online, and discovered some copies of this album going for ridiculous prices. A sealed copy sold for 1500 smackers(how? why)???!
I am wondering if my opened, played, yet mint copy is worth considerably more than I paid for it, It sounds wonderful, warm, and clean, but not as clear as the classic reissue.... I find on the original pressing, the individual instruments seem to occupy a more well-defined space, if that makes any sense. I want to do more listening to this wonderful album, but I am almost afraid to play it for fear of doing damage! I realized yesterday that I actually put spindle-marks on the label, I think it may have been unplayed before I bought it!!!
If anyone knows what this album goes for, and if it is very rare or not, any info would be well appreciated.
fwiw, the record store guys were 'concerned' when I returned and told them it is actually stereo and not mono.
thanks, Harv