There are a lot of Top 10 or 10 Things to do/not do lists out there but this seems to be one of the best for SEO. It is the Search Engine Journal, so they should know best. A lot of dealerships out there should take careful note of item number 6 and then call your vendor. 10 SEO Tips article on Search Engine Journal.
I was speaking with a dealer today, and they are completely new to the arena of online marketing. They were asking a lot of questions about best practices, and one of the tools I provided for their education was a presentation developed by Autotrader.com. If you're new to selling/promoting vehicles online, or even if you'd just like a brief refresher course, this tutorial should be well worth your while. It discusses everything from the importance of writing good descriptions to taking appropriate photos.
