Stewart Warner Tachometer 430636

I was digging through a box of what seemed like mostly junk in a little shop yesterday and I saw this old tachometer. My 9 year old loves car stuff so I paid $3 for it thinking it would look cool on one of his display shelves with old cars. Out of curiosity I looked on eBay and found the following auction. It is the exact same tach and is selling for $75! Wow, I had no idea it had any value, just thought it was cool. Here is the eBay link
http://www.ebay.com/itm/Vintage-stewart-warner-diesel-tachometer-4000-Rpm-/370463257719?pt=Vintage_Car_Truck_Parts_Accessories&vxp=mtr&hash=item564154b477