Keep in mind that it doesn't cost them much money. They would just have to run a server that checks and signs some transactions, so we don't need a large incentive. For instance, a merchant or exchange operator could offer this service for free because it protects themselves from double-spends. Given that double-spends have already happened on the network, this should be a significant incentive.

Alternatively, auditors could also charge for their service. Fees wouldn't have to be high. Given that the auditor is only doing very little work, fees would be no more than ~ €0.10 per commenced 1000 tx. This fee would be payed in advance because including a seperate output for that purpose in each tx would be too expensive.