旧币 literally means "old currency"; it is not referring to any currency in particular, and without context it's hard to be sure. I believe the usage of "旧币" is only to tell the reader that, despite using "元", it is not referring to the modern renminbi, which has never had a value so low that old, off-season hawked clothes would cost thousands of it.
The most likely answer IMO is the old, first series RMB. This had a brief history between 1948 and 1955, after which it was revaluated at a rate of 10000 old RMB = 1 new RMB.
Of course there are numerous historical Chinese currencies called "元", and if you keep their value in mind, it could have been during the late civil war's hyperinflation, but since currency valuations were so much in flux during this period, this answer is less likely.
Even less likely are the various pre-Republic yuans, since they were pegged against stable, high-value foreign currencies like the Japanese Yen and Mexican Peso (no sarcasm, these were both stable and high-value in the early 20th century).
By the way, I would pay more attention to the usage of "元" in the prices than the phrase "铜钱". The latter used to be real currency but is probably being used as a synonym of "money", just as "钱" used to be a unit of currency but is now used as the word for "money" today.