I am considering getting Tienda, and want to use it on my suite of sites. I haven't decided yet whether I want the same product listing on all sites yet, however. Will I be able to have it either way? (i.e., have a slave site using the same products/etc OR have a slave site that just shares the functionality, but a completely different set of products)

Thanks!

When you share an extension (including tienda) this share the MySQL tables used by the extension and sometime also require to share the users.

This is the case of "tienda" that share both the users and the tienda tables.

The decision to share the user is something that must be decided when creating the slave site. It is not possible to convert a slave site with "standalone user" into shared users as all the "user id" and information must be identical between websites.

So if want decide to share tienda later, you need at least to have the Joomla users shared between the websites.

Thanks Edwin!

So, if the Tienda code is shared, then the data (products, etc.) needs to be shared, too? Is there any configuration which allows shared code, but separate users AND separate products?

NO.

The PHP Code (the application) and the data (DB content) are two differents things.
This is like a word application that is able to manage several word document. Each word document have their own content but managed by the same word application.

Here this is the same.
You have the same PHP code for tienda but as you are connected on a different DB then you have different content.

By default each website have their own DB or at least different table prefix to have their own content (user, menu, forum, tienda, ....)