I just say i am making dry ice hash and i need 10 pounds.
I still don't see why one would mention how it is used. If I was asked nosy questions about the purpose I would find a different place to buy it.
And when i am done with it i leave it in a bucket in the garage to gas off.
Or leave it in the grow tent (in small amounts) to give the plants some additional CO2.
Safety standards for CO2 are in the range of 5,000 ppm (or more, even) over an entire work day, that's over 10 times the normal concentration.
If you left 1 pound of dry ice to sublimate in an airtight room 10 by 10 feet, it would lead to maybe 1% of the volume in CO2 concentration (about 10,000 ppm). That's still safe for several hours, some regs say an entire work day. If the room is only slightly vented, you'll never reach these levels.
Also, it's not toxic, so even if you exceed the levels you might feel a little short of breath, but you won't die. It's not CO (carbon monoxide), that's a different story, that stuff is super-dangerous at much lower concentrations. Has nothing to do with this, however.
One should probably not leave it in the bedroom with windows closed and then go to sleep. Otherwise, I wouldn't sweat it.