Specifically Expected Answer :

One-unknown way of thinking :
>! If R is the number of rich buyers, then "3R + (**12**-R) = 2 times **12** + 3" <br> which gives fractional R. <br> Here the second variable is "D = Dozen" (either 12 [Common] or 13 [Bakers]) so the correct equation is "3R + (**D**-R) = 2 times **D** + 3" <br> which gives integral R, when D is 13.

Two-unknowns way of thinking :
>! If R is the number of rich buyers and P is the number of poor buyers, then "3R + P = 2 times **12** + 3" & "R + P = **12**" <br> which gives fractional R. <br> Here the third variable is "D = Dozen" (either 12 [Common] or 13 [Bakers]) so the correct equations are "3R + P = 2 times *D* + 3" & "R + P = **D**" <br> which gives integral R, when D is 13.

Bonus Clue was :
>! In the last line, "Basically, there is one EXTRA unknown." is redundant, pointing to **"one EXTRA"** or 12+1.

Core of the puzzle :
>! Baker + Dozen = Bakers Dozen : refer http://en.wikipedia.org/wiki/Dozen and follow.

Hope all clues were highly visible , yet hidden !!!!