pseudonym58 the sales person should be able to recommend a particular time when there's less likely to be other people around
But @Josephine titled this thread "private seller" so I assume she means she'll be doing to someone's house, and there's no sales person. It's easy to imagine that a person selling a piano out of their house might feel uncomfortable leaving someone alone in the piano room, I assume that's what Josephine is worried about.
@Josephine when I was piano shopping and visiting the homes of private sellers, they mostly showed me where the piano was and then went into a different room. So I think you'll be fine. But it's ok to ask as well.
The other thing I want to suggest is that in this situation, you are the buyer and that puts you in a position with a certain amount of power. I say this not to suggest you be rude or something, but rather because hopefully it will help make you less nervous. I have terrible stage fright and shaky hands when I play for others, but when I was piano shopping, I was fine. Usually I'd have a minute or so of adjustment but then I was able to just focus on the piano and how it sounded and felt. I knew I wasn't there to impress anyone (and mostly, you'll never see them again) and it wasn't a performance but a chance for me to evaluate the piano.
And regarding the one where there's a piano teacher selling the piano, piano teachers are the best because they understand that you're learning and they don't judge.
Hopefully thinking of it this way will help you.
Good luck and let us know how it goes!