I recently bought a used ER25 collet set and 1" diameter straight-shank arbor, aside from missing a few of the larger collets, it seems to be in good condition. The long taper in the arbor was galled, but not badly. It looks like someone forgot to tighten the arbor nut enough, and the collet spun in the arbor when used under power.
If one rotates a collet in the arbor by hand, it catches. The arbor is hardened, and a file was not successful at cleanup. (The steep taper on the arbor nut is perfect.)
To repair the arbor, I tore off a strip of 220 grit wet-dry sandpaper, wetted it, wrapped it around one of the collets with the rough side out, and gently sanded the inside of the arbor taper. This worked quickly and well, preferentially removing the galled metal that protruded above the level of the taper.
The arbor is not marked with a maker's name, but appears well made. The only markings are "025-EXT" and "100". It was claimed that the set was made by ETM, and one of the collets is so marked, but I doubt that the rest of the pieces are by ETM.
Joe Gwinn