I am purchasing 2 used marble countertops that I need to cut down to size. Is a wet saw critical, or can I use a diamond blade (recommeded by blade manufacturer for marble on the label) in my circular saw and do dry cuts? Thanks.

The damond dry saw can do the job if you go slow. However, it will be very dusty and the cut will be rough and probably not 100% straight. You will need to do some polsihing with a wet slurry. You could call around to see if any counter top companies would do the cut for you but obviously that is more expensive than just buying a blade. These folks will have everything you need if you DIY.
Ceramic Blade, Blades For Ceramic, Marble Blade, Diamond Blades for Marble Tile

You could have someone take a water hose to it, take the nosel off and just have a steady stream of water covering the top as you cut, just don't get shocked.

What is used to join 2 marble tops?