I myself am a student of metallurgical engineering( dealing metals specially steel) n once a sheet is cold worked (crushed...stretched extra) the residual stress in metal increases drastically n for denting further cool work is done (hammering etc) which further increases stresses in the sheet hence making is weak..therefore there is nothing as 100% perfect repair...micro-cracks appear which became stress concentration zone..when an accidented car undergo another collision these micro-cracks are from where propagation of cracks/defects initiates resulting in total failure...
Therefore, be very careful buying JDM used cars...its better to buy a smaller car within ur budget then a bigger accidented car for shoo-shaah...
The people in foreign countries have knowledge abt such things therefore these car are exported to 3rd world countries....
Ur Life is IMPORTANT..May ALLAH PRROTECT ALL..Ameen