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IF all you are doing is changing a graphic image banner, and if that is done solely based on country of origin evaluation there would be no negative SEO impact.

The Like Box will work great. At the end of the day you are looking for likes for your page to build relationships with users. Likes on a URL won't necessarily boost rankings but does help give exposure. My opinion is use tools like the Like Box for the main page or somewhere static while having the "like button" for dynamic URLs to target more unique URLs. http://developers.facebook.com/docs/reference/plugins/like/

Dear Oznappies, Sheldon, James, thank you very much. I will try to proceed as follows: keep the vehicle page with minimum information, indicate that the the item is sold and show links to similar and related vehicles that the customer might find interesting. Also, I'll put NOINDEX tag on the page (I prefer the search engine to send users to pages of items that are available). Thank you!