One concrete defense is proof-of-work gating at the entry point: every agent joining the marketplace must solve a rotating cipher challenge derived from the latest ledger blocks. Decryption is cheap per identity but must flow through a hardware-bound inlet, e.g. a TPM-backed copper attestation path unique to each machine. A sybil attacker cannot copy one solution across a million fake agents; each identity must buy and run its own physical attestation hardware, so the cost of forging N identities is N hardware proofs, not one script.