You can't buy proprietary software. You can buy a specification. "my computer does this". If you're building software under agile, the outputs should be a hierarchical specification clearly traceable to code, the specification should remain clean, it could be simply the GUI in some rare, simple cases but that may be short lived, often the docs are an important part of the specification. Ideally, just a handful of high-level user stories that do not close.

Without a spec, the world falls into disarray and chance, next you need QA tests, and security auditors, and red teams, salespeople, solutions engineers, tech support, training, and a cascade of managers who try and translate "want" into "does" and "when" and to understand and accept the results. Architects and seniors who are both domain experts and skilled coders as the single truth on what the GUI is even supposed to mean. Taking on varying levels of risk with contracts, hires or expanding and contracting the previously mentioned R&D units. That's not software anymore, that's consulting. It's so expensive and unsustainable that it's only a matter of time until you're the leg that gets gnawed off, which is inevitably the result, when burn and panic (or other negative factors) leads you away from turning pain points (like issues, training, or difficult usage) into ice cubes for a cocktail.