Old coins if its made of Gold and you will sell it with GOLD or if its silver and you want to sell it with silver then it must be of the same wieght and the selling process must be in the same meeting ..which means that the person who will buy this gold coin (lets say a golden coin) he must pay the equivalent wieght of gold in the same meeting. for the Hadith of the prophet : (peace and blessings of Allah be upon him) said, "The gold for the gold, the silver for the silver, wheat for wheat, barley for barley, date for date, salt for salt, similar for the like, and hand in hand. So whoever adds or seeks to add, then he has used riba. The recipient and the giver of the transaction are the same". (Reported by Muslim, #1584.)
if the coin was made of anything other than GOLD and silver like if it was made of copper or iron or anything else than silver or gold then its OK to sell it by any price as far as the seller and the buyer agree.. because the reason of ribaa isn't here in this case because these old coins are not considered currency nowadays
AL-Sheikh ebn Othaymeen has answered a similar question and also AL-shiekh al munajjed...
Arabic sources are available if needed?
Wa/ Assalamu alaykum wa rahmatullahi wa barakatuh.