Resale values on Volkswagen AG’s diesel cars in the U.S. have taken a tough hit in the weeks since auto maker’s emissions scandal emerged, falling an average of 13%, or about $1,700 per vehicle since mid-September as car dealers stay away from the tainted vehicles, according used-car pricing guide Kelley Blue Book.

“Early indications from auctions are that dealers are more hesitant to buy the VW diesel units,” KBB’s lead product analyst Tim Fleming said. “Used car values typically decline at this time of the year, but the...