In my opinion, I disagree with your seller. Maybe she meant that you should pet her more when she's out in the open, not hiding under the bed. She will eventually get more curious and realize that she is not in danger. Then she will not always be under the bed and start checking out her surroundings. Have patience, this will take a few days. Don't run after her; let her come to you on her terms. Speak quietly and calmly to her so she doesn't get spooked. She will come around!
I think this too. In addition, I might put a small dish of food and water within sight of the bed to coax her out sometimes. She may be tempted by it when you're not in the room and then eventually even when you are. It just takes a little time for new kittens to adjust. She'll be a loving companion soon enough.
And she sure is adorable!