different people have different purpose for advertising,
some advertise more for branding/awareness purpose and didnt focus too much on conversion rate.
most advertise to make more sales.

For a start craigslist is definitely good since it is free.
eBay has the huge amount of potential buyer.

you will need to know who is your main target audience.

- local buyers?
- within US?
- international buyers?

Find advertising media that has access to your target audience.

Find a way to keep track of your advertising dollars how well it converts,
advertising online vs advertising locally in your newspaper/bulletin/magazine.
online local classified, offline local classified.

eg. local newspaper, when they mention 5% discount code NEWS, you know they are from newspaper.
online sites, when they mention 5% discount code SITE1, you know they are from that particular site.

go to local auto show, local car workshops, car clubs, give out your name cards, offer them discount for referring.
words of mouth works very well.

you can find out if local car clubs have any online community or forum, talk to the club owner.
advertise in small local car online active community is more valuable than expensive ads in large international online community.

list yourself in business directory, yellow pages.