You can offer them at sedo.
As far as I know the market is still in it's beginning, except some very popular one letter types as ü, ä, ö and good words using special characters. If you talk about 1 or 2 characters domains and want to make good money on it, it would advise to sit on it for some years. Because I think they will bring nice sums of money in the future.
btw Anybody know what the best sold IDN domainname is so far?