It's kind of like buying a car, new is good, but there's always a chance of warranty issues, small defect or just working out the bugs. Buying used can get you some great deals, or could get you dumped with a lemon.
I would say where you get it from matter more than what you get. If it's new or even used from a reputable business who will support you after the fact, then you're probably safe with either used or new.
Similarly if you're looking on the equipment exchange on here, people with very high feedback scores that are all 100% (with the occasional unreasonable -) are generally quite accurate in describing condition and take their reputations quite seriously and won't rip you off.
What kind of rifle are you looking for, wood/blue hunting rifle, something tactical for the range, .22 for plinking, etc? There are lots of the sponsors up top who will have what you are looking for.