The only manner to determine that an eBay user is a registered seller is by making a 'minimal' VerifyAddItem request as seen below. This boiler-plate request is a fully functional VerifyAddItem call with the least amount of call overhead possible for a valid API request.
<?xml version="1.0" encoding="utf-8"?>
<Location>San Jose, CA</Location>
<Title>Seller Test Item Title</Title>
If the user is not a seller, the following error message will be returned:
<ShortMessage>Credit card not found.</ShortMessage>
<LongMessage>We're sorry, but before you list an item, you must create a seller's account by submitting a valid credit card through our secure form.</LongMessage>
If the user is a seller, the standard VerifyAddItem Response will be returned, showing the Listing Fees, and a 0 value for ItemID.
Secondly, the GetUser API function returns a number of circumstantial indicators such as having a credit card, PowerSeller field, etc.. These indicators are useful for determining the specifics of a given eBay user, though, they can not be used to determine, with absolute certainty, that a user is a registered seller. Your specific application may be able to provide logic based on GetUser return values for application-specific user determinations.
* Please be aware that the minimal VerfiyAddItem request is the only bullet-proof manner for determining if an eBay user is a registered seller.