there is slime even in london. i haven't been slimed by any camera dealers, mainly because i have not purchased much equipment here.
i once went to change money for polish currency (zloty) and danish currency (krones) at an area with many moneychangers (king's cross). one particular money changer gave me a rate slightly better than usual for zloty, slightly below par for krones.
i said fine, your rates are alright, i will go draw money.
when i came back, the krone exchange rate stayed the same, he claimed that there had been MAJOR MARKET MOVEMENTS and that the zloty had somehow become a lot more expensive in the 3 minutes i stepped out to draw the cash.
naturally, i told him what i told of him with a gesture and left the place.
it is not fair to overgeneralise. if you actually notice, you are probably less likely to get ripped off in the less touristy areas of poor countries which are peaceful. the people there are simple, they lead simple lives, money is not a major concern to them - hence they don't resort to such means. on the other hand, once you have tourists and money coming in, things change.
slime has more to do with greed, and development introduces new things, new concepts, consumerism. of course, there are also things like laws and enforcement to ensure that unfair practices do not happen all the time, but i think it is more than safe to say that being careful all the time will never give you the wrong result.