I just removed and re-installed a toilet due to some bathroom redecorating work. Everything seemed fine, but now there is a very tiny leak from the plastic nut that connects the water feed line to the bottom of the toilet. The nut is tight, but there a leak from the inner circumferance. What is likely causing this, and what do I need to buy to fix it?
Probably just repalce the water line that runs from the nut down to the supply valvbe. The end that is inside the nut has a flat disk that acts as a gasket. It is a cheap and quick fix.
In most toilets there is either a cone washer or an o-ring and a brass washer in that fitting. IME, leaving out the brass washer pretty much guarantees a leak. Also, the cone washer may not be seated right or is too deformed from previous use to form a good seal. Remember, the threads only hold the thing in place, they don’t form the seal in this type of fitting.
First, disconnect the supply tube from the bottom of the tank and make sure there is no piece of debris preventing proper seating between the supply tube end and the ballcock inlet fitting. Wind the nut back on finger tight plus a quarter-turn. See if it leaks. If it does, try giving the nut just a smidge more torque. Obviously, since it’s plastic you don’t want to go all Mighty Joe on it, or the leak will be larger, suddenly. 
If that doesn’t work, go get a replacement flexible toilet supply tube. They can be either braided stainless on the exterior or white plastic. Lengths are typically 9" or 12". Female toilet thread on one end-the other has to match whatever you’ve got-often 3/8" compression female, but could be 3/8" or 1/2" IPS male, depending on your particular shutoff valve and supply plumbing.