Many good responses have already been made, but I'll try to add a little extra value.

1) a) I own a few Nikons. One of them is a D300 dslr. How would I determine this camera's shutter count?

Lot's of ways since Nikon publishes shutter count in the EXIF data, if the file has not been modified in software. Different EXIF viewers may call the record by different names, and some EXIF viewers report different amounts of information. There is also software that can retrieve the information directly from Nikon cameras, although one regular contributor to UHH categorically denies that.

b) Is the technique the same for all makes and types of cameras?

No. Canon does not put shutter count information in EXIF data. It can be accessed in various ways, the easiest by software, on such package is EOSmsg (http://eosmsg.com/). It is not free, costing between $5 and $10 for each individual camera for as many readings as one wishes and supports many Canon, Nikon, Pentax and Sony cameras.

2) Could a seller *adjust* a camera's shutter count so as to misrepresent the usage of the camera? Is it impossible to fudge the count or merely difficult? Would "refurbished" play any role in a camera's shutter count?

To the best of my knowledge it is not possible. It is possible to edit the EXIF data in an image, but takes effort and deceit to do. Refurbished cameras - at least from the manufacturer - come with reliable shutter count information.

3) I read somewhere that the average life expectancy of shutters on recently made, good quality cameras is around 100,000 clicks. But I have seen cameras with much higher shutter counts than 100,000 rated as condition Excellent. What IS a reasonable shutter count max? 100,000 is not a bad number to work with a few may 75,000 and some 150,000 or more for high end models. It is simply an MTBF estimate (Mean or average time between failures) and no guarantee. It will depend upon your usage of course, and occasionally being lucky or unlucky, but if you capture 10,000 images per year then the camera should last about ten years, which is a long time in electronics. Shutter replacements are not horribly expensive if needed.

4) As far as I could tell, KEH gives their used cameras an adjectival rating (such as excellent, good, bargain, ugly) but do not give shutter count -- at least, not as I have ever seen. I wonder why?

I would speculate that their margins are not huge, so the cost of checking every camera would eat into their profits, and also mean that some cameras would probably languish in their inventory or have to have a significantly lower price.

5) Many Ebay sellers do not reveal shutter count. I even ran across an experienced seller with a 99.8% rating who wrote, "I don't know the shutter count." I avoid sellers who do not disclose click count, but I wonder if that is a fully valid criterion on my part?

I would speculate (again) that many do not know how to do that. Also there is the time constraint and the liability from providing too much information to a buyer, since anything deemed inaccurate is grounds for a return which erodes profits.

I hope this has been of some help.

I have been using Picassa for the last ten years and love it. Never had any problems.
I just bought a refurbished d7200 to go with my old d300s. Love them both.
My problem is that in the last two weeks, any pictures taken with the d7200 and shown in Picassa looks like it has a pink sheet of paper infront of the picture.
That doesn't happen using the d300s and the same sd card.
I can import the photos into LR and then export them to another file. Those pictures, when view in Picassa, don't have any tinge at all. Just when they come first from my d7200.
Any help?

TJ Keegan