De Niro's character gave Finn a huge headstart though.
People usually follow like sheep and a wild success for his first exposition was a great way to ensure the word of mouth about him would be super positive. People are people, they just follow the original impact, whether positive or negative.
It's particularly true of art.

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I thought this too, but I reconciled that perhaps he simply offered more than the other buyers in order to get the paintings. The other people in the film seem to generally like his work, so I get the feeling Deniro's character was probably much more of a fan of Finn's, for personal reasons, than the other buyers at his show and having the money he just bought them all at more than the asking price.
But yes, it is a valid complaint that this isn't explained more.
However, remember, at the end of the film Estella says that she's heard about him, and knows he's been doing well. So it wasn't just Deniro's character that supported his success.

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