there's all kinds of threads about this on rebelscum. basically, the MR sabers were made in china with questionable plastics -- "not entirely stable", this plastic seems to be decomposing over time. this is pandemic to the entire MR line, FX and LE sabers alike -- it seems they will all break down over time and turn to 'black goo' like this. humidity and sunlight seem to be accelerating factors... although... I live in a very dry climate, and I've had the same problem with mine. (for example last christmas, I gifted a brand new Luke eSB FX, 'new in the box', to my wife's cousin as part of a 'secret santa' gift exchange. the grips on the saber were tacky, and this one had never been exposed to sunlight nor humidity. it was 'mint in sealed box', and it was still decomposing over time).
in the end, there's not a lot you can do about it -- it's a known issue in the collector' community -- if anything, it proves the saber you bought is an authentic MR and not a counterfeit :P hehe. you could try to replace the grips as suggested, but this will effect the resale value -- and you;ll probably find that the grips are not glued on, as you might like/expect; IIRC, the plastic grips include molded anchors that go through holes in the hilt to hold the grips in place -- this will be a major PITA to remove :(