Rhys, Craig, and Dave have all given you excellent advice. This forum is an incredible resource as the members are both knowledgeable and willing to share.
While evaluating a picture from a technical standpoint may be a more concrete process, never forget to collect what you enjoy and handle the items with care. There are no right or wrong answers to many of the questions posed.
In the beginning of the Yee/Fogel book, the 4 C's are described. I think they help define the basics of what most photos collectors look for in a picture.
Content
Clarity
Contrast
Condition
For myself, Content is King. Clarity and contrast are next. Condition only matters if it's an image I know there is more than one copy of and I can get one better. For many images only a handful have been seen, so condition becomes less important.
I am torn about editors marks. Sometimes I think they add to the picture, sometimes take away, and sometimes don't matter. It all comes down to personal preference. I also think, for me at least, I don't remove them because I don't want to take a chance of damaging the picture.
Here is one of mine, bought in a Yee auction last year, that I have debated whether to remove the editors marks. I think I got it for a very reasonable price, at least in part, because of the markings.
