Despite global chip shortages hitting new car sales, Marshall said it expects full-year profits to now exceed £50 million, up from expectations of at least £40 million announced in August.
The firm, led by CEO Daksh Gupta, said its used vehicle values have seen an "unprecedented" 26.3% rise over the past seven months.
Lookers also said it expects full-year profits to be "materially ahead" of previous expectations after seeing "excellent" margins on both new and used cars in the past quarter.
Shares in Marshall gained more than 8%, or 17p, to 225p, while Lookers was around 5%, or 3.3p, higher at 64p.
Semiconductors are needed to create car features like parking cameras and lane control, and global chip output has slowed amid supply chain and logistics issues in the pandemic, stalling production and supply of new cars.
Consumers face long wait times for new models and this, along with pandemic savings and wariness of public transport, has helped the used car market soar.
Today both dealerships cautioned that they expect chip shortages to continue into next year.
Marshall said delivery times for new vehicles have been "significantly extended", and despite having a "strong" order book for new cars, Lookers cautioned that "there remains material and increasing uncertainty as to the availability of these vehicles".
Analysts at Peel Hunt said the issues fuelling used car margins "are unlikely to end on 1 January".
The updates come a day after FTSE 100 engineering group Melrose warned it is being hit by the global chip shortage.