Most solid-fuel stoves (wood-stoves) built today and in use have fire bricks, or refractory bricks around the sides, bottom and some even on the top of the firebox.  These are made from clay and they crack.  This is nothing to be worried about it, it is a normal process as the bricks are constantly being taken from hot to cold temperatures which causes expansion and contraction to occur on each of these temperature cycles.
This is nothing to be alarmed at. Infact, when you think about it, it’s something to be expected.
Replacement of the bricks, rope gasket seals, stove glass and baffle plates are all consumable items in any wood-stove.  The brick may or may not how sign of wear but still may be weakened through.  This is why a Chimney Sweep should check the bricks and replace them if needed and why it is a great idea to always have a WETT Certified Chimney Sweep provide your WETT Inspection; especially in a home purchase!   Most sweeps will either replace the bricks as part of the service or charge a nominal fee of perhaps $5 or $6 per brick; with a small upcharge if bricks need cutting.  So brick replacement should not be something to worry about as it is simply part of normal service and maintenance process of owning a wood-stove.  Always hire a WETT Certified Chimney Sweep!