Undeniable signatures cannot be passed on from one person to another. If Alice verifies Bob's undeniable signature, she can't prove to Charlie that the signature is good. She can claim it is good, and assure Charlie that it is good based on her own reputation, but Charlie can in general not be convinced unless he verifies it himself directly with Bob.
This is the standard reason given why undeniable signatures can't be passed on. And it is correct, as far as it goes. But the conclusion, that "in general" the trust cannot be passed on, while technically correct, is not of pragmatic consequence. I'll start a service to perform any undeniable signature verification, even ones for money. I'll perform the verification, and then make an attestation that I perfomed the verfication and whether it succeeded or failed. I sign this is a regular digital signature, the kind that is infinitely duplicable. Only a few such services need exist to assure the public of the results of a signature verification. True, there is a layer of mediation here, but of what practical consequence is that? In fact most transactions are mediated already. If I expect to make money charging a dollar per verification, and if there are some who will publish their experiences of the verification, that reduces the total income I can expect to, oh, say, the logarithm of the size of the market. In other words, why bother? Eric