be careful not to pull too hard on the pullstarter on those HPI .15 star engines. short and fast 4inch pulls using your fingers, its not a lawnmower. If it doesnt want to cooperate, the recoil spring inside the pull starter is usually damaged or the one way bearing engaging the crank is gone.
You might also have a 'flooded' engine, where too much fuel is accumulated inside the sleeve.
I'm guessing since the nitro car was at a garage sale, it might not have been stored in the best of conditions.
If you need to take apart the pull starter, find a friend who has experience in this. whatever you do, do not try to pull out the recoil spring from inside the starter or it will go (boing-boing-boing) richochet all over the place. Rewinding the spring with yr hands isnt the best way to do it, but it helps to see how it works at least temporarily.