If you can't tell the difference, you really shouldn't be looking on ebay, the difference of these 4 frame designs is very obvious, and all are designed for a very different purpose / riding types.

You need to identify what type of riding you will be doing, then look for a frame for it, but buying a complete bike will almost always be cheaper and simpler, especially with the ever changing standards MTB's have.

For a Specialized S Works, with out giving more details, it could be a Venge or an Epic, or a Stumpjumper or a Roubaix etc, just as much as a AM frame or CX, more info needed. S-Works is just Specailzed word for their race line, and covers many bikes, bike parts and clothing items.