Removing malware and rootkits
First of all boot into safe mode and use a root kit remover (Such as TDSSKiller from Kaspersky,) to check whether there is any software that will persist across malware scans etc.
Then use a traditional malware removal tool such as Kaspersky's free malware scan to remove more standard malware.
Regardless of whether you do this you should attempt to regain control of your account through customer service and issue charge-backs (Note: This may have repercussions depending on the terms of service.) Then change passwords for any accounts that share the password and check the registered email to see if you have any notices from ebay (Such as password changes.)
Edit
If you want to remove all possible malware running Malwarebtyes won't do it, you may need to run that as well as a root kit remover.
The purchase order coming from your device does indicate that it was compromised. Did anyone else have physical access to the device, does anyone else know your password or are you running any form of remote access such as Chrome remote desktop or SSH?
Edit 2
In reply to Xavierjazz no they will not have card details, accounts only display part of the card number in order to stop this happening, there should be no need to cancel the card.