Although it’s technically possible to tell HTML to use a “sans serif” font (ebooks being based on HTML), ebook readers all seem to ignore this from my own testing. Besides, much of the point of e-readers is that the user can choose his or her preferred font for reading. To this end, all e-readers have two fonts: the main reading font and a monospace font for things such as code blocks. The user may be able to choose between variants for these two fonts, but they are determined by the reader or e-reader, and not by the book itself.
Now, it is in fact possible to embed fonts into ebooks, so that the e-reader displays the font embedded in the ebook instead of the default font of the device. Scrivener does not allow for this, however, so you would have to use a third-party solution if you wanted to embed fonts. There’s a good reason for this: to embed a font, you’ll need a licence, and that you will have to pay good money. You can’t just embed the fonts available on your machine - they can be used for many things, but not for redistribution in the form of being embedded in an ebook.
So, for instance, if you want Arial to be included in your ebook, you would need to buy a licence, for instance from Fonts.com for £60 per book title (that’s cheap compared to many fonts). If Scrivener made it easy to embed fonts, we’d have users inadvertently breaking licence agreements.
All the best,
Keith