"Artists Beware"

If you're uncomfortable with the subject matter requested, that is a perfectly valid reason to not accept a commission, but unless you expect each person to die with all the original artwork they've collected, you cannot reasonably expect a physical piece of artwork you've done to never be resold. Medical expenses come up, tastes change, and if you see your artwork has gotten more than you initially sold it for, just take that as information that people are willing to pay that much for your art, and you can consider raising your prices. Picasso didn't stop drawing because he saw people reselling older artwork he drew for higher prices, he raised his own prices!