Credits earned from listings (where you are the seller) are the only type of credits that can be sold. All other credit types (obtained from offers, goals, purchases, for checking in, etc.) are not eligible to be sold. Furthermore, credits earned from listings in the Gift Cards and Bitcoin categories are not eligible to be sold.
For accounts that are able to sell credits, your Sellable Credits (seen in your credit activity on the website) is the amount of credits that is immediately available to be sold.