Marbles were easily disposable kid's toys. Machine mades sold for 1 cent - or less. They were produced in staggeringly huge quantities.
Identification, grading and value are best and most easily learned at shows where you can handle marbles, look at them from multiple angles, study color, transparency, opacity and variations between same types. Value is driven sharply by type, grading, current popularity, availability and the motivation of the seller. Learning grading from photos is like running a footrace carrying 80 lbs ballast: you could (sort of) - but why try to do it with such a pronounced handicap? Handle enough polished marbles and you can do it as well by feel as you can by vision.
You can pick up a HUGE amount of experience and insight in two days (and nights!) at a good show.