That might be a bad choice, I just looked at TWRA's 2012 fishing regs, first of all you would need to have a valid fishing license, next you have to use legal gear types, next you need to check if the stream or river you want to collect from has any special rules pertaining to capturing the said fish. Finally, if there is no management or protection in place on these darters they would fall into the category of a "Class C" bait fish, and the rules surrounding those includes:
1) they shall only be used in the water from which they were harvested
2) they shall not be possessed away from the waters from which they were harvested
3) they shall not be imported into Tennessee or exported from Tennessee by anyone
4) They shall not be sold
Oh and to sell baitfish, if they were of a class that could be sold, you would need a bait dealer license.
Also, since there are many diffrent species of darters, some of them ESA listed, and for the most part they look the same, you could easily end up selling an endangered species on the internet, which would be a very bad situation.
And Don't get me wrong, I'm in favor of wild caught ornamental fish-if the proper precautions and regulations are in place-which in this case they are not-and thus collection + sale is illegal.