In general, displaying a large field of many options is less preferable than showing a smaller subset of higher-level categories with clear labels, allowing users to drill down into successively more narrow categories (assuming we're talking about a "select-a-product" type task.)
If there are multiple paths or options involved in a checkout flow, then generally you want to progressively reveal advanced options as they become necessary.
That's a vague answer, so maybe it's helpful to consider the context of use for most of these volunteer cashiers - will they have continual exposure to the UI over time, such that they'd be able to learn some elements? Are they likely to have only intermittent contact with the UI, occurring at far-apart intervals?
Also, what kind of point of sale system are we talking about? PC running a web UI? Windows OS application? Touchscreen kiosk?
What kind of data entry is going to be involved? Will volunteers need to select merchandise from a fixed menu? Simply enter in prices? How often does the inventory change?
Sorry to answer with so many questions, but the right answer might depend on the details. If all else fails, you might want to find a retail outlet near you with self-checkout kiosks (many grocery store chains here in the U.S. have them, as well as Home Depot. IKEA tends to have them as well, though not in all locations). I think the use case is similar to what you're dealing with: users who come into infrequent contact with a point of sale system and need clear and concise choices throughout the checkout process.