The gallery owner refers to a general agreement when discussing van Gogh's painting style, but doing so isn't a flaw in this argument. It's perfectly reasonable to cite expert consensus as a reason to believe a certain style is characteristic of a certain painter. In this case, the general agreement is good evidence.

While experts can lend authority, there's no need to cite experts for this claim. The gallery owner can reasonably state that van Gogh often painted a particular subject just on the basis of personal experience. So could anyone who looked at several van Gogh paintings of this subject.

This isn't something the argument does: the gallery owner never refers to profit, or any other specific reason to buy a painting.

The only evidence the gallery owner gives that the painting is by van Gogh is the subject and the style, and these could both easily be replicated by someone else. The gallery owner's failure to show why we should believe the painting is a van Gogh and not a replica is a flaw in the argument that the painting is authentic.

It might be in the gallery owner's interest to sell this painting, but we don't know if that's why the gallery owner argues that the painting is authentic. There's no clue in the argument that the gallery owner is unconcerned with the truth, and we can't assume any particular motivation.