If you're not sure that your idea will be profitable and you're afraid the investment won't pay off, your main goal should be to minimize cost. There are a few ways to check whether our plans match reality:
consider the dropshipping model before stocking up and locking money in inventory sitting in a warehouse. Although it gives you less control and a lower margin, the product investment shrinks to a minimum,
use cheap SaaS platforms or the Allegro/eBay/Amazon channel — although it requires concessions, it lets you save on big technology investments,
do solid research — find stores similar to your idea and try to estimate their sales scale — e.g. by monitoring them with tools like Allegro Tradewatch, or placing orders with the competition at the start and end of the month and inferring volume from the difference in order numbers,
study user demand by entering keywords describing your idea into Google Ads Keyword Planner, which reports the search volume of specific phrases on Google. 