I've actually been thinking of giving Domain.io a shot, as my portfolio is becoming a bit unwieldy, especially with all the TXT codes needed, and the requirement to change nameservers multiple times. That, and ability to give the potential buyer multiple venues to purchase from is a real bonus.
I do wish there was more detailed info on the Domain.io site, such as exactly which sales venues are supported, is there a limit (3?) per listing, etc.