As others have said, if you're going to learn about and sell prints and reproductions, the first thing you need to do is buy a good loupe, and the second thing you need to do is learn how to carefully disframe what you buy.
The value is going to depend much upon just what they are; that is, what kind of print or reproduction they are. You cannot study comparables until you determine that.
Take them out of the frames, to look for information which may be printed on them.
Look at them through a loupe. If it's a traditional, non-mechanical, lithograph, you'll see a sort of stippled pattern all over but any dots are rather small, vary somewhat in size, are in no set pattern. If the dots are uniform in size and in a regular pattern, it's mechanical photo-offset lithograph, a mass-produced reproduction.
If you don't see dots with the loupe, what do you see? Maybe you have an original work. Maybe you have an intaglio print of some sort, such as an etching. Or it could be some kind of relief print, such as a woodcut. Perhaps it's a variety of stencil print, such as a serigraph.
You need to learn to recognize all these because, as I said, they have a huge impact on value.
There is abundant info on the web, but this is a good place to start:
http://www.graphicsatlas.org/
