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I recently purchased a Link A-12 bubble octant on ebay, from the seller "balloonron".  I just wanted to share my experience, since this is a small enough community that one guy's experience might actually matter.  I know I constantly ended up on this list when googling around for a bubble sextant for purchase, so I'm posting on the off chance it helps someone else. 

Balloonron seems to have gotten ahold of a batch of Link A-12s, which he has restored to perfectly functional condition.  I think there are still a few for sale on Ebay. I bought one last week.  I hadn't used Ebay in years, and had reservations, but at a nice savings over the ones Celestaire was selling, I thought I'd give it a try.  (Which is not to disparage Celestaire, I've had good service from them as well.)

Everything about this experience has exceeded my expectations.  The instrument (in original case, with original paper manual) is beautiful, the optics are perfectly spotless, collimation looks to be excellent, the bubble chamber lighting mechanism has been updated to eliminate the issue of C battery size changing over the years, a full adjustment tool kit and bubble servicing kit (including a tube of fluid and spare tiny steel ball seals) has been included, a lighted loupe for easy reading of the scale was included... etc.  After placing order, the seller initiated frequent contact inquiring how I'd like certain options etc.  Backyard sights with this instrument are an absolute pleasure, I've gotta say. MAN does it beat a bowl of water or other artificial horizon!  ;)

I'm not trying to sell you anything. This is not a solicited testimonial.  It's just, customer service like this is rare enough these days that it deserves a shout out.  I know I'm just a random guy on the internet, but if you're in the market for a bubble sextant, I can highly recommend "Balloonron" on Ebay.