In this case, it's rapidA, fitting the gender of la venta, which gives you a clue it serves as an adjective in this case. If the subject of your example was ella instead of él, she would still run "más rápido", altough she would be más rapida.
As for the use of "rápido" as an adverb, my Spanish dictionary lists it as being able to serve as an adverb only in Americas, while it says it's adjective only in Spanish Spanish, with rápidamente being the adverb version in Spanish Spanish. Good to remember depending on which version of Spanish you want to learn. :)
Does anyone know if this is a good sounding sentence in Spanish? Because I don't think I'd ever say thins in English. "The car was sold quickly"? Yes. "The car sale went well"? Yes. "The car sale went fast"? Maybe. "the sale of the car" really bothers me, but "the car sale" doesn't seem like a good translation because it does not refer to the sale of a particular car...
Because something sounds funny in English gives little indication of whether the Spanish is odd Spanish, IMO. There are MANY things in Spanish that need odd translations into English: all the tener expressions, the hacer expressions, the gustar expressions, many reflexive verbs, multiple negatives in a sentence, and more. ;)
But I hear ya. "The car sold in a hurry" works for me, but try that in Spanish and you'll probably end up with a passive or false passive, and the car will be selling itself. ;)
I'm a native spanish speaker, there is more than one way of saying this in spanish here is a few....first of all Carro and Coche are equally used in the americas...."el carro se vendió pronto" = "el carro se vendió rapido" = "se vendió rapido el carro" = "se vendió pronto el carro"