Want to sell illustrator CS3 found at yard sale

If you purchased used software and the original owner didn't complete a transfer of license to you, then you have no legal rights. If the product was still in the sealed box, the first person to install it becomes the legal owner, and has to comply with the transfer rules as linked above. Since you've now installed the product you cannot realistically sell it on eBay, as their sales terms and conditions don't allow for the Adobe license transfer policy to be implemented first.

Or here:

http://helpx.adobe.com/x-productkb/policy-pricing/transfer-product-license.html

License Transfer http://kb2.adobe.com/cps/152/tn_15281.html