I would suggest you to go for carparts.com as it is more easy to remember and include your keywords as well
I agree with the reply above from an easy to remember perspective. However, on purely SEO grounds the "-" is better. This is true for a variety of reasons. A domain name that includes keywords assists primarily since those who point links to your site sometimes use your URL, not because the domain name has the keywords as most people assume.
In the case of "car parts" it won't matter. Now if you're running site that has a domain which "could" contain multiple keywords, then the hyphen would be better. That's why Experts-Exchange.com uses the hyphen in their domain name... otherwise they'd be ranking for "Expert Sex Change" as well and given the technological inclination of their audience, I don't think they'd want the kind of traffic that people looking for sex change operations would bring!
If both were available I would go for the one without the hyphen, just a better chance of type-in traffic.