What’s the hukm on coin collecting? Can they be sold at higher value?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
“Coin collecting, also called numismatics, is the systematic accumulation and study of coins, tokens, paper money, and objects of similar form and purpose. The collecting of coins is one of the oldest hobbies in the world. With the exception of China and Japan, the introduction of paper money is, for the most part, a recent development (meaning since the 18th century). Hence, while paper money and other types of notes are collectable, the history of that form of collecting is distinct from coins and largely a modern phenomenon.”
As collector coins are considered to be items that are rare, unique and antique. They will not be considered to be from the regular currency (coins, notes) that are commonly used today.
Therefore, they may be sold at a higher price owing to the fact that they are now a rare, collector’s item, and not a regular coin in circulation today. 
Only Allah knows best
Written by Maulana Tahsin Alam
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham