In many countries there is already a market for buying and selling real world identities of penniless students and homeless alcoholics to use for nefarious ends, e.g. financial fraud or registering shell companies. Given enough financial incentive, Proof of Identity is insufficient.
Proof of identity with a social network backing of people vouching for each other then. As long as ids are limited we can track and assign trust in a secondary market. Anyone pulling stunts like you mention is just selling out their credit for a one-time small gain. Of course that will happen still, but it can be disincentivized financially - and has been in irl societies less desperate or with more attached to their social credit
