Used encoders aren't things people sell often... And when someone puts one up... they go in a few minutes. Just not the type of item you upgrade.
Of course... I just upgraded my ok MK64 to an IPAC4 and took me all of 1 person I mentioned it to before it sold... And I only did that because MK64s cant be programmed in the non-front facing window... And I wanted to be able to have my frontend configure my control panel automatically...