The problem with not sealing the inside is moisture getting into the wood, which can cause the wood to move/warp, and can actually cause some surfaces, like glass, to break under the stress of the movement. This problem wouldn't show up immediately, so you would be running the risk of having a bunch of pots in customers hands that start warping down the road if the inside is not sealed. I use rattle can spar, and I spray both inside and out to try to get a good seal on the whole pot. Like others have mentioned, a good penetrating oil finish will help prevent this as well.