Back in the dotcom days I was involved with a business auction service with built in escrow services.

Are you in the US? If you haven't, get a good lawyer that knows about escrow services. Many states had various rules about what requirements are needed to call yourself an escrow. I know initially we had to not claim to be an escrow in California because you needed to be a member of a certain trade association. This is decades out of date info but expect lots of hoops.  As crypto has become mainstream I would only expect more hoops.

As to marketing the service you'll have to dox yourself so people know who they are trusting. Also be very upfront about where funds are held. Also be sure you can publicly say you are using them, one bank didn't like us using their name in any fashion without it being cleared by their legal.