Check for Unique Seller Name

Ensure that when a new seller is registering, their name is unique. If the seller name is already taken, prevent the registration and show a message indicating that the seller name is unavailable.

Start by locating the seller registration entry point and the existing seller persistence and validation logic. Trace how registration errors are returned, then verify that a duplicate seller name is rejected with an unavailable-name message and that a unique name still registers successfully.

Written by the indexing model from the issue text.