QOL: Muramana / Seraphs's Embrace Exchange

So I was playing an ARAM game, and bought a tear to start, then accidentally built it into Manamune, not realizing until it upgraded to Muramana.. Since it was upgraded, it would sell for 0 gold..

Obviously I messed up. But why can't we get a sum of gold from it from selling, so I could atleast buy another item. Or why can't we just exchange it for it's AP Counterpart, maybe make the exchange cost a bit of gold.

It just makes no sense to me why it would sell for 0 gold. And Riot has implemented the 'Exchange' idea for the starting jungler items already.