How do you sell Pokemon cards on eBay?

Yes, but it is frowned upon by the gaming community. As long as we are talking about MMO's. Steam or Xbox Live accounts you might be able to sell (I don't know about those) but that would be a bad idea as they are linked to credit cards generally.

As for selling it, eBay might not be your best bet for selling it. With a simple google search I found a website which sells accounts on a large scale and will buy yours from you (see the related links). Look into more sites like that for a better sale is my suggestion.