That way you can just keep track of how it's doing and if you can make some profit from it then you could even just trade what the bot trades and profit and make the sale for free. Otherwise, if the entire business model depends on "this bot is good, trust me" then I am sorry but why would I pay anyone based on that.
this.
to prove trading bot is effective, the maker can easily just give history of trades, we aren't lacking any platform that give such data, the bot maker could also easily set up copy trade account so that people can observe how effective the trade is, but truth is, majority don't do that, because they know that trading bot can help simplify trade and increase the odd but not a guarantee whatsoever that the user will make endless money from it.
if it's that good, the bot maker will use the bot for himself and make billion, but they didn't because they know the bot can be wrong too.
when using bot, just consider it like using a tool, bots can't magically print infinite money.