The unfinished ones on ebay are easily finished to look very presentable. Follow the bouncing ball:
Get some Neatsfoot oil, and some Fiebing's TanKote, both available at Tandy or Leather Factory or probably your local Crafts Store.
Using a sheeps wool dauber apply sparing coats of the neatsfoot oil to the unfinished sheath. Allow the oil to penetrate and level out. Do this about three times. After the last application has leveled out apply the TanKote with another wool dauber and allow it to dry completely. You now have a darker, finished sheath that has a slight sheen and is water resistant because it has been sealed. IT IS NOT WATER PROOF ONLY WATER RESISTANT!!!
The smallest bottle you can buy of the two products will treat probably 25 or 30 sheaths, but it's not expensive so that's okay. The ebay sheaths are probably about $10.00 for a lot of three plus postage, and they are pretty good sheaths. I think I finished a couple for Codger that he sent along with some other stuff, so I finished them out of curiosity and as I remember they turned out pretty good. Get Codger's opinion.
Edited to add: If you want to dye it a different color, the buy some Fiebing's dye the color of choice and wet the sheath really wet. Apply the dye and then let it dry very thoroughly, then proceed with step one above.
Paul