Unfortunately there's little help we can give when a seller is permanently restricted.
Al we could have suggested is that you ask eBay for details, but as you have discovered they may refuse to provide further information. This is frustrating, but there are often good reasons why they cannot do so. You accepted this in the user agreement.
As has been said, it may be because you were selling as a business on a private account (private sellers are only allowed to sell their own, unwanted possessions). But I think eBay usually first advises sellers of the need to upgrade to a business account.
It's entirely possible that it was through no fault of your own. Accounts may be restricted simply because eBay has detected an association with another account that has been restricted. This can be could be someone living at the same address, or potentially who has logged in to eBay using the same connected device. This is done to prevent restricted sellers from simply starting up again using a family member or friend's account.
Unfortunately, there appears to be no appeal.
