I think part of it is human nature. It's essentially the reward system at play. The buyers drugs arrive.. and that excites them into thinking that since they have been "rewarded" their drugs, they should "reward" the seller with feedback while they're in the midst of being so ecstatic. Then they finalize and figure that even if the product is good (notice I didn't say mind blowing), they have an obligation to heap praise and 5/5 etc because 'it could have been worse'. And maybe part of them thinks somehow the vendor will give them more/better/ next time if they kiss a little ass. Also part of the issue the amount of kids on SR.