Can anyone explain to me why there are such big differences between the selling price and the trading price?
I know that trading212 makes money from the price difference, but this difference is way too big.
They don’t make money from this. It’s illegal to inflate the spread for profit.
This is a liquidity issue.
This is a small company in which the trading volume is not great. The spread is always larger in such cases as it will take longer for the market maker find sellers to match to buyers, and vice versa. It is hard for the market maker to know that he will be able to sell the shares for close to what he paid for them, so he has to quote a greater spread to give himself protection.
You are incorrect in thinking that Trading 212 makes money from the price difference between buy and sell. In the Invest account the entity who makes money is not Trading 212, but rather the market makers who Trading 212 accesses via its partner Interactive Brokers, as @Donald_Duck has also said.