How Huemill keeps trade fair
In a single-player automation game, a cheat only changes your own save. In Huemill, the things you make end up in other people's factories, so a cheat would reach them too. That's why the game is built so that the server, not your PC, decides what actually happened.
Your PC sends actions, not results
When you place a building or list something in your shop, the client never tells the server "I now have 500 steel plates." It sends the action you took.
The server keeps its own copy of your island. It runs that copy forward to the moment of your action, then applies the action there, with the same code your PC runs. If you can't afford the building, or don't have the items you're listing, the server rejects the action and your island doesn't change.
Every machine gets the same numbers
For the server to replay your island, its simulation has to produce exactly the same result as yours, bit for bit. Floating-point math can come out slightly differently depending on the CPU and the compiler, so the simulation doesn't use it. It works in integers (fixed-point where it needs fractions) with its own random number generator, and it advances in fixed steps, 20 per second. Tests feed the same inputs through the simulation and check that the results match.
Your island's clock runs only while you play
Your island advances only while you're connected, and the server won't accept it running faster than the game's top speed of 3x. You can't fast-forward past what the game allows, and you can't claim time you weren't there for. The flip side is that nothing decays and nobody gets ahead of you while you're offline: your island simply waits.
Trades are settled on the server
Buying, selling, cancelling a listing, and sending a gift are each settled in the server's database as a single transaction. If two players try to buy the last ten units at the same moment, only one of them gets them. Neither island changes until the server has recorded the trade, and a resent request is never applied twice.
Trading moves money between players; it never creates it. New money enters the world through NPC requests, which the server issues and checks against its own records.
What visitors see is real
When you visit another island, you're looking at a snapshot that the server has checked against its own copy of that island. A modified client can't show off a factory it doesn't have.
Who you are comes from Steam
For the tests and beyond, you'll sign in through Steam. The server confirms your sign-in with Steam directly and decides who you are from that, not from anything the client says about itself.
A modified client gains nothing
Anyone can modify a client. But whatever it claims, the server checks the result itself, so a modified client can't fake items, money, or time. The most it can do is send the same actions any player could.
Letting a program play for you (a bot) is a separate matter, and it will be prohibited by the terms of use.