Andrew here, thanking Gil Skillman for his reply to me (ope-l: 738) and
 continuing the discussion:
Yes, Gil's right, the 2 of us have difficulties "translating" between our
 respective theoretical vantage points.  Given that my references to 
passages later in Ch. 1 of Capital, and Ch. 2, were indeed meant to
 "illustrate the sense of" Marx's argument, and not as a logical "proof"
 of the initial argument, I'm happy to focus on the paragraph Gil singles
 out (6th para. of Ch. 1, top of p. 127 in Vintage/Penguin ed.).
As to Gil's paraphrase of Marx, I didn't use the word "commonality"
 instead of "equivalence" of commodities to challenge Gil's interpretation.
  No I take these to be the same.  I wrote that "a commonality to these
 commodities is disclosed by examining this particular form of an 'exchange
 relationship'."  The key word is "disclosed":  whereas Gil maintains
 that the equivalence or commonality is *established* by the act of 
exchange, I maintain that Marx does not say this, and indeed says the
 opposite.  Thus, when Gil says that the members of an equivalence set
 need share only the commonality that placed them in this set, I agree,
 and I maintain that this particular commonality was that which Marx
 searched for; but this commonality is not the fact of exchange, and
 so we must look elsewhere for the basis of that equivalence.
I agree that a quarter of wheat will not have only 1 exchange-value (in
 terms of any particular other commodity) unless the law of 1 price
 holds.  But this isn't relevant--Marx is considering this one particular
 quarter of wheat, not a different one.
Marx writes that since the wheat exchanges for a series of other 
commodities, which "each represent the exchange-value of one quarter of
 wheat ... [the other commodities] as exchange-values [must] be mutually
 replaceable or of identical magnitude."  Gil says that "of identical
 magnitude" doesn't follow.  But he seems to contradict this, next
 saying that *any* good, even one that isn't a product of labor, would
 be seen to be of the same "identical magnitude" by exchanging for the
 wheat.  I agree with the latter point, and therefore not with the
 former.  This does not invalidate Marx's argument, since he's asking
 what *commodities* have in common, not why goods *exchange*, what allows
 them to exchange, etc., etc.
It should also be noted that the equivalence in Marx's example is a
 quantitative one, and thus that the commonality or equivalence or
 identity or whatever you call it is a quantitative one.  This is not
 true of any and every equivalence--Gil is right here--but I can't see
 how mutually replaceable quantitities mean anything other than that
 the quantities are themselves equal to one another.   
If A is worth $10, B is worth $10, C is worth $10, then A, B, and
 C are each mutually replaceable entities, or of identical magnitude,
 as expressions of the worth of $10.  How is this not so?
Andrew