Many buyers are looking to auctions to acquire Mr. Fordjour’s works, and resale “secondary” market prices are rising steeply. Last month, a 2017 painting sold at Phillips in London for $169,000. “The auctions reflect where the speculation is,” said Candace Worth, an art adviser based in New York. “There is a clutch of younger artists that have become flash points,” added Ms. Worth, who singled out Mr. Fordjour, Julie Curtiss, Tschabalala Self and Loie Hollowell.