Probably Stolen water trading in the Water Merchant start in the Steam demo includes water testing and purification equipment. Test water, treat it with the tools you have, retest it, and only then judge the sale against the cost of containers and power.
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Probably Stolen water trading: first batch
The water line carries liability when you sell a quality you have not checked. The exact treatment required depends on your water source and equipment, so use the current reading rather than assuming every container begins the same.
Take one container from your available supply and test its quality with the starting kit.
Apply the filter or other purification tool available in your current setup.
Test the treated water again. Repeat only if the reading still misses the quality you intend to sell.
Put the treated water in a usable container and compare the buyer's offer with the full preparation cost.
A pitcher filter and UV lamp can improve the setup, but the final purity depends on your source and machine configuration. Retest the batch before pricing it.
Probably Stolen bottle printer upgrade
In the September demo's UPDATE 049, electronic parts unlock upgrades for the Bottle Printer, including normal and large bottles. Inspect the printer's current upgrade prompt for the electronic-part quantity before committing them. Printing an empty bottle addresses container supply, while testing and purification address the water inside it.
Price the complete sale
A high offer for filled water can still be a weak trade if the container, power or treatment consumes the margin. Compare the offer with all three inputs, then keep enough cash for the next rent cycle. The rent survival guide shows the reserve check to make before buying machinery or more water stock.