Among collectors, there are people who collect coins and banknotes which are considered rare or unusual. Often these would be older currencies which are no longer produced (although they may still be considered legal tender), but this could also include currencies which are currently in circulation which are considered special in some way (e.g., coins with an unusual printing error, or banknotes with interesting serial numbers).
Such currencies are often bought and sold at values significantly above their face value.
Is such a practice considered riba? If so, is there any halal way to trade in such currencies that allows the buyer and the seller to capitalize on their actual (rather than face) value?