Can a 110VAC to 240VAC transformer be used safely in reverse?
There is no theoretical reason that a power transformer can not be used as either a step-up down transformer. There is a problem with the frequency. If the transformer is designed for 240 V at 60 Hz output, the 240 V side should have 240 V at 60 Hz. If 50 Hz is applied, the ratio of voltage to frequency should be maintained at 240/60 = 4. 240/50 = 4.8 or 20% above rated. If the transformer is rated 120:240 V 50/60 Hz, and the link says it is, it would be ok. At 20% above rated voltage, the iron could saturate causing the transformer to operate hotter than it should. The reduced frequency does reduce eddy-current and hysteresis losses. That might be enough to offset the increased losses due to increased magnetizing current. The linked data sheet does say 115/230 volts, not 120/240. However, the two are generally considered to be equivalent.

I would not hesitate to use the transformer as a 220-240 to 110-120 volt step down transformer. I would not resell it as such without checking the IEC certification details and any other standards that might apply.