Idk... kinda feel like why buy a cracked frame when there are many non-cracked ones out there? Then again, and I could be wrong, but even if it split another inch down the tube while riding seated (which I'd think is unlikely, as you're not putting stress on the exact spot of the crack in that scenario), it seems like the worst thing that could happen is your seat post gets a little flexy and you ride home standing up while thinking about what your next frame purchase will be.

I guess it depends on just how good that deal is and how risk averse (or not) you are.