Thought provoking article!
What can one say… “yip!”. Massive missed opportunity. Millions of people accumulating hard-to-fake trust and reliability, in a world where those are incredibly valuable and hard to prove assets.
Can they now “un miss” it?
The competition is not
You’re right that while there’s similar-looking competition (“log in with MasterCard”, “trusted by AirBnB”), people won’t want to link their credit card to website log-ins, and, as you say, AirBnB might not wish to let a competitor link to their reputation system, so the door is slightly open for a middle man who happens to know you are trustworthy.
The detail of some space on the log-in page
It‘s hard to convince millions of websites to add a “log-in with ebay” button: Google, Twitter and FB already own that. A start would be a super-simple code library on github that that lets site masters automate log-in and lets users add a “trusted at ebay” badge to their credentials.
Trust without disclosure
Trusted anonymity would be another play: Let people use their ID and a challenge string from the site they want to authenticate with to generate a response which ebay can verify as authentic and bearing a certian level of trust-worthiness, but which reveals nothing about the user’s ID, simply that an authentic user created this string. Apple/Android Pay where ebay is the users “bank” account.
Allowing people to sponsor others would be valuable too: You’re new on commerce site X, but are my friend, so I link my trust to yours — any losses to be taken from my ID, perhaps showing “trusted by four A★ users”).
Bitcoin all round
A low hanging fruit, it seems to me, would be for ebay to give every trusted user a Bitcoin wallet and default to turning conventional payments into bitcoin and receiving bitcoin.
That would have massive impact — a reverse Thiel/Musk play: paradoxically making ebay into what Pay Pal was supposed to be — an anonymous currency and TrueNames realm — with a nice marketplace alongside.