I'll take a shot....
1) As commodities these items have exchange value, which is derived from that labor.
2) In order to analysize this disparite labor we must homogenize it. In this way we can speak of labor in the abstract allowing us to deal with all forms of labor.
3) Well, it seems to be a logical construction to me. As far as 'historical basis' goes, I'm not quite clear, unless we are speaking about labor as a historical necessity of all humans.
4) Each phrase builds upon the previous, all commodities are the product of labor, this labor is apparently insubstancial, for the purpose of analysis all of this labor is equal.
5) This substance has no physical form yet it is 'real'(this confuses the shit out of me, how can the insubstancal be real?). In any case this substance is abstracted labor. Marx said that this abstracted labor is a real thing in society, it's basis really.(Yeah, I still got problems with this but crystals is a good metaphor.)
(Repeating myself it seems. But it seems that Marx does it a lot, driving home the point. I realize that with each slightly different repitition there is nuance which sometimes(often?) goes over my head.)
