The law of one price (LOP) can be found in Marx vol. III p.865.
The diffrence of course is interpretation of the LOP. The neoclassical
interpetation would be just a single price that every producer sells his
product, as is the case with perfect competition. Marx's interpration is
a rather narrow spectrum of prices that the majority of firms in an
industry are forced to sell because of competitive pressures. Of course,
some firms can sell at different prices because of location, customers
loyalty etc.
The law of one price, with the above qualifications, I guess can also be
found in the writings of classical economists.
Lefteris Tsoulfidis