I assume you are talking about your Auctiva eBay Store, correct?
If you click the 'store' tap on the menu bar underneath the blue Auctiva header, you can see the things that can be customized for your store. Unfortunately, we don't have any feature that supports welcome messages based on the user id of the visitor to your store.
Now if you have an Auctiva Commerce Store, that is a different product and you should post your question in their forum.
Sorry I can't be more help.