Yea on karma for one sec from a Buddhist perspective , as BoscosFriend said it is based on the intention behind an action, also many people falsely perceive karma as a purely mystical mumbo jumbo thing but it goes beyond that. It's not a big hourglass that adds a bit more sand to each side based on what you do, it relates to your actions and the positive or negative effects they may have. There's more too it than that but anyway.
Back on track, i don't personally think there are any shocking side effects with short term marijuana use, i think it can actually be quite beneficial for some. The main side effect that i can see though is that a user can ( and most probably will ) become mentally dependent on it, and the side effects of long term use then are basically those listed above. So you may need to convince your friend not to sell it, ( i may have interpreted it wrongly but is that what he intends to do ) as I'm not sure about where you live but here i know that it is illegal and going to jail or getting a heavy fine is a side effect no-one should have to cope with.
Back on track, i don't personally think there are any shocking side effects with short term marijuana use, i think it can actually be quite beneficial for some. The main side effect that i can see though is that a user can ( and most probably will ) become mentally dependent on it, and the side effects of long term use then are basically those listed above. So you may need to convince your friend not to sell it, ( i may have interpreted it wrongly but is that what he intends to do ) as I'm not sure about where you live but here i know that it is illegal and going to jail or getting a heavy fine is a side effect no-one should have to cope with.