PLEASE PLEASE PLEASE can support help instead of ignoring! I have requested numerous times for help with these sandbox issues. I can not list any items with my sandbox users as I keep getting an error stating that the accounts need to be selling accounts. So how do I convert a sandbox user account to a sandbox selling account? I also need these accounts converting to business accounts (please don't just paste the support URL as this is where I get the error saying can't convert to business accounts as they are not selling accounts.) I also still need these accounts linking to Paypal sandbox accounts (I've already requested this weeks ago via the support url.) This complete lack of support has brought a complete halt to my web application development.
I think you need to link a PayPal account for them to become selling accounts is my guess... I'm also in the same position as you trying to get my sandbox eBay and PayPal accounts linked so that I can test selling.