Nissan Motor Co. ’s Leaf electric car has been a big seller for Atlanta car dealer Pat Hoban over the past three years, thanks to its low monthly lease price. But as those car leases are beginning to expire amid cheap gasoline, the vehicle is becoming a bit of a headache.

Mr. Hoban expects between 100 and 150 of the leased vehicles to be returned to his Capitol City Nissan dealership on a monthly basis over the next two years as their leases expire. The problem: used Leafs aren’t attracting much demand.

With gas prices down 33% from a year ago, and buyers cooling toward electric vehicles, some auto makers are offering deep discounts or attractive leases on battery-powered vehicles and plug-in hybrids. Nissan, for instance, slashed the price of a new Leaf by $6,400 in 2013 and is now offering a $199-a-month lease, or $3,500 cash back and 0% financing for 72 months, on brand new Leafs.

Buyers who also get a $7,500 federal tax credit on purchase of a new Leaf, see little reason to shop for a preowned model and some worry the expensive batteries could have to be replaced. “Used Leafs haven’t really taken off,” Mr. Hoban said. “There is really no incentive to buy a used one when you can lease a new one for less.”

This has driven down resale values of plug-in electrics including the Leaf and General Motors Co. ’s Chevrolet Volt, representing another hurdle for auto makers trying to boost sales of alternative-fuel vehicles.