its all about reducing the cost per gram! that is the main reason i vape! - ok second best reason!
if you smoke it, you get medicated for a few hours then your done. if a gram is $X then you can be medicated for a certain amount of hours per gram (on average, depending on all sorts of variables of course). however, if you vape your meds then you can extend the number of hours you are medicated per gram by a significant amount! the ability to vape at certain temps, for certain effects, is also a nice benefit!
so whether your selling the 'leftovers' back or ingesting them yourself - it works out the same, your reducing your cost per gram. except that its WAY cooler to sell the dealer back his own stuff after you "got your moneys worth out of it", i.e. you got
!