What fraud does Tiernan commit in the novel, The Witch Elm?

Tiernan is an artist at the gallery where Toby works. He has trouble marketing his own work, and thus decides to exhibit the work of underprivileged youth artists. Tiernan attempts to pass off his own work as that of an 18-year-old named “Gouger.” Toby discovers this, but assists him in this fraud until Richard discovers it.