How can I move a 500 lb wooden table top into my basement down a flight of stairs?
That is a big piece of wood.  There are three obvious approaches:


Get some professionals to do it.  A grand piano weighs more than that (and is more awkwardly shaped).  Professionals will have the experience and equipment to do it safely, and the indemnity insurance to pay if anything gets damaged.
Slide it gently down the stairs (possibly manufacturing a slideway out of a plank or two, or just cover the steps with blankets).  Use ropes looped around the table to restrain it, and possibly a block and tackle to reduce the load until it is manageable.  (With the block and tackle, you can probably arrange not to have anyone standing below the table as it goes down - which strikes me as desirable).
Don't try and move the top as a single element.  There is no way it can be made of a single lump of solid wood, so buy the elements separately, and assemble them at the bottom of the stairs.