I've considered something like this, and I'd like to offer some pros and cons.

Pros

• You actually get your own people elected
• Want a policy implemented? Probably you can do it
• A well-organized community defense that's set up early could be effective against counterintelligence and also stand as a useful example of what the left actually wants when it says "abolish the police"
• Since profit is not the main goal of the city, a lot of transactions can be tax exempt, greatly reducing your need to financially support US imperialism
• The civil rights act prevents many kinds of employment discrimination. Political orientation is not a protected class, so you can most likely keep your city as free from capitalist infiltrators as you please
• A sufficiently large city of anti-capitalists could influence nearby elections by encouraging residents to rent in swing districts, potentially negating gerrymandering in that state (you would need a LOT of people for this to work)
• The city acts as its own advertising. Anything that goes right is its own argument for your governing style

Cons

• The city acts as its own advertising. Anything that goes wrong is its own argument against your governing style
• Obviously, not all of the users on this site are US residents
• While cities mostly get to make their own rules, different states have laws that limit what laws a city is allowed to pass. Even if you find loopholes, generally nothing is stopping the state from closing them
• For the greatest chance at gaining control of the city, you must pick a small town that has next to no infrastructure
• Small towns tend not to be in economically advantageous locations
• If you get any kind of traction you will be targeted for harassment and/or assassination by the FBI. If you got national attention, the national guard could even go after you. Corporations most likely would make your life difficult.
• While cities and non-profit organizations do get certain tax advantages, employed individuals still have to pay income taxes
• Cities are not allowed to print money like the federal government. Combined with federal taxes, the net effect is an outflow of dollars from the local economy that must be offset by exports to a market that has a consistent supply of USD (not necessarily exports to a foreign country). It's just a lot easier to organize an economy how you want when you also have control over your own currency.

Edit: With the amount of people you would need to make a highly functioning economy with people who are apparently willing to move to a random city in the US, you might as well try to take over a larger city that already has infrastructure. Unfortunately, this also means that you will have a hard time actually getting elected as the city's political machine will be set against you from the start.