@Account Closed to flip homes you are required to have a contractor's license in MN or have a contractor complete the work. Here is the information for you to read, it falls under "builds or improves for resale or speculation" https://www.dli.mn.gov/busines...
MN DLI requirement are for general building including pulling a permit for an egress window or any general building permits. Plumbing and electrical work has to be completed by a licensed plumber/electrician. You can't do the work even if you have a contractor license.
The inspector is doing his job and letting you know the law at the same time, some cities seem a bit more lax on this. Right now it sounds like he is tipping you off to issues and being a pain but if he sees you replaced all the plumbing without a permit that is a quick way to get a final stop work order and him require all licensed contractors.
Other flippers make money by having people who can work at a reasonable price and they find good deals. You know what they say, "flippin' ain't easy"