Either the person got access to your Auctiva userid and password, or your eBay userid and password. Change your password on both sites. Cancel the listing on eBay for the camper, and file a support case at eBay as well requesting the fee refund for the cost of listing the camper.
In the future be careful about selecting a difficult to guess password, and do not reply or click links in emails that request your password. Some criminals have been sending emails that copy our emails, but change the links to use their websites to try and steal passwords, that is called phishing. Once they have your password they do something like this.