Is this rim still safe to use?
Sorry, those rims are trash (or wall art).
I'd chase down the seller and demand your money back, and report them on whatever sites they were advertising on for selling items in a known dangerous state.
The crank may be small now but it will propagate in both directions and get bigger. Cracks on thin section aluminum cannot be welded as the heat will destroy the heat treatment state of the metal around the repair.