"The thief does not want to sell the gems."
Translation:Fur gemmas vendere non vult.
vult is indicative, specifies a 'fact', it is translated as 'he/she/it wants'.
velit is subjunctive, does not specify 'fact' but rather things more like 'conditional', 'potential', 'opinion', etc. There are plently of different uses of the subjunctive. Duolingo translates velit as 'he/she/it would like'.