Brock is quite correct grammatically. But - logically/practically your second version is a little redundant, as it is obvious that pipe that is cut into individual 'pipes' will have a length. So, the first version is the version you will normally hear.

Thanks!

Karen's explanation was what I had in mind when I saw these two sentences. I have another question though: should it be ' Copper pipe is sold in LENGTH' and 'Coppper pipes are sold in LENGTHS'?

Brock's answer is correct, the expressions have essentially the same meaning. However there is a very subtle difference between the sentences. "Copper pipe is sold in lengths" suggests that there is one very long piece of pipe for sale (singular), and multiple pieces or lengths of pipe (plural) are removed or cut from the single piece of pipe when someone buys pipe. It implies that the price of each piece of pipe relates to its length. "Copper pipes are sold in lengths"suggests that there are many ( plural) pieces of pipe for sale, and also implies that the price of the piece of pipe is determined by the length of each piece

Pipe is both singular and plural. This is the same for other words in English, such as 'hair', 'deer', 'fish', (many more). So, remember : is=1 are=2+ If you speak of copper 'pipe', you must use 'is'. If you speak of copper 'pipes', you must use 'are'. Both have identical meanings. Ahh, the joys of words that have singular and plural meanings :) Hope this helps!? :)