The prefixes are taught in pronunciation (click here). They are GE-, BE-, HER-, VER-, ONT-. When verbs come with these, remember they act like regular verbs, but in the past tense they do not get a GE- past tense marker added. It's like wearing two sets of glasses -- no thanks!
So KOOP is to buy (a regular verb) and VERKOOP is to sell (a prefixed verb). Look at these present - past examples:
Greg koop die boek. > Greg het die boek gekoop.
Greg verkoop die boek. > Greg het die boek verkoop.