Objectively speaking, MIB is Mint In Box - a toy that is in mint condition that is inside its box. The may or may not have been handled since packaging, but that's not a prerequisite of MIB. It's a descriptor of its condition, not its handling history.

MISB = Mint In Sealed Box, so by its very nature of being sealed it would most likely never have been handled since packaging. The difference between MIB and MISB? Some bits of sticky tape. I guess MISB can fetch higher prices because it's kind of a guarantee that the toy hasn't been handled, whereas MIB doesn't necessarily imply that per se.