As you will have discovered by now, RKs have virtually no commercial value. They do, however, have great SOCIAL value.
Spend some time showering strangers with Random Kindness and see how much you and they enjoy it! Scatter the little rainbows in unexpected places for others to pick up and use as well.
RKs spawn from gardening crops (not sure about herbs, but I don't think they get any). Even if your crop garden is stingy, you can get at least one RK per week and usually more. When my garden is feeling generous, I can end up with 2 or 3.
There is also a quick and easy quest in The Tower that nets you an RK, and retaking it to get another RK costs only 100c and less than 3-4 minutes of your time.
Reselling them--or attempting to--is not ever going to be a good way to try to make a profit. Most people use them as intended and distribute Randomly Kind energy and mood boosts to others, or give the RKs away to other people who are planning to use them to give out boosts to others.