Do you mean outside ads -if so you can get suspended! I tried using Clickbank links in ads on Ebay and after 7 days the auctions started getting deleted - but on the flip side I made $400 in commission from the sales!
No I dont think you can add Adsense to your auctions. If you notice at the bottom of any search page on Ebay, there are Adsense Ads displayed buy Ebay will get the revenue.
There is another way....thats to sign up as an Ebay affiliate, then look on Craigslist for someone whos after a car and how much, search on ebay and use the Ebay Affiliate tools to generate an URL, then email that URL to the people on CL looking for the car? They buy through your link, you make 20% of purchase price - its something like that anyway.