I'm a big defender of Chinese industrial capability to manufacture a superior product, but in 2019 the fact is that unless you're buying 100K+ units, factories operating outside the tech industry all sell their products on such razor thin margins that the buck or so saved by skipping the hardening step on your rollers is enough to make the difference between profitability and selling at a loss. Anything sold directly (ala Ebay) likely fall short of contract manufacturing specification. The difference here is a seller who stands behind their product. Raw materials make up a huge part of the total manufacturing cost in China, and you don't want bargain rollers in your mill. Source: 20 years experience in product development and sourcing, including cutlery and machinery.
Bronze bushings are typically specified where operating speeds are high enough and duty cycles long enough to heat up the porous, sintered material sufficiently to bring the impregnating oil to the bearing surface. If these conditions are met there will be essentially zero wear as the shaft rides on a cushion of oil. In shorter duty cycles and dusty conditions like a grain mill, the bushing will likely have a shorter life compared to a ball bearing. Cost is the driver here, and more important than bushings or bearings is their quality, i.e. a high grade bushing may outlive an inferior grade of ball bearing, though that is unlikely in this particular short duty/dusty application. Bearing races will attract dust but they can be blasted clean with solvent and re-lubricated whereas bushings will need replacement.
