Has "per piece" been used properly?

Single examples of printed items such as newspapers, magazines and books are usually called "copies". Also I have made a few alterations in the text you posted. I am not sure what the text in red is supposed to mean - it looks like it was translated by a computer program.

Yesterday a vendor at a main street in this city sold the special issue for 15 yuan per copy, while the original price was just 5 yuan. Some passers-by who bought the issue complained that it was too expensive. What did the female vendor reply? She said, "the space of this special issue to increase its value is capacious!" "If you don't buy it now, you can wait and will certainly pay more for it!", she added.

You could just say, "for 15 yuan each", and "this special issue is sure to increase in value". I'd use "woman" instedad of "female" if you insist on putting in that rather rhetorical question in, or just say "What did the vendor reply? She...", since the "She" established the gender, so you don't need to specify it otherwise, which seems a little awkward.

I agree with Monterey Jack.

MJ has answered the text in red for me.
Thank you.
And thank MJ.

"15 yuan apiece" would also be more idiomatic.

what is this supposed to mean?

I wonder if oristar was thinking of "There's a lot of room for this special issue to increase in value", but that's sort of an idiom, and you can't replace those words with "the space ...is capacious".

I worked it around in my heads a few ways that could work, but none seemed quite right.

something along the lines of "there's potential for this special issue to increase in value" or opportunity or ...

original - does that mean what the paper normally sells for? or does it mean that this issue was priced at 5 yuan by the producer, and this vendor marked it up to 15 yuan?

It's not clear from the original, and the meaning is quite different.

Yes, you've taken the words out of my mouth.

Well analyzed.
It is "normally".
Thanks