When a fixture's original glass is broken or missing, you have to replace it, and the honest way to do that is worth being clear about. Fitting a matching lamp shade isn't faking anything, so long as you don't pretend the new glass is the old glass. The goal is a fixture that works and looks right, described plainly for what it is.
Start with the distinction. A fixture with its original glass intact is one thing; a fixture with a replacement shade is another, and both are legitimate, but they're not the same, and a buyer deserves to know which they're getting. The problem isn't the replacement. The problem would be selling a replaced shade as the untouched original. When we replace a missing shade, we look for glass that suits the fixture in period, style, size, and condition, so the piece looks whole. Sometimes the replacement is genuine vintage glass of the right type; sometimes it's a quality reproduction shade. Either way, we say which. This is different from faking age. Distressing a new shade to look old, or passing a replacement off as original, is the line we don't cross. Matching a period-appropriate shade honestly, and labeling it, is the line we work on.
Salvager's Note: A replacement shade is honest as long as it's called one. Ask whether a shade is the fixture's original glass, a period vintage replacement, or a reproduction, and expect a straight answer. All three are fine; being told which is what matters.