You should never buy a secondhand helmet. Make sure any online helmet prices which seem too good to be true are UK model helmets, from a reputable dealer. You may not have any problems, but you might need backup in the event of a problem.
Please be careful when buying helmets,You should treat your helmet like your toothbrush.
Would you use a strangers toothbrush? Your helmet moulds to suit YOUR head and will not provide adequate protection if it has shaped itself to someone elses specific head shape.
This will start from the initial time you wear your helmet and for the first (approx) month to three of regular use will quickly settle in to suit you.
Comfort linings and cheek pads compress very quickly and affect fit instantly, the protective inner shell of the helmet takes a bit longer but will always be different from person to person.
The outer shell is the least fragile part but must be unmarked and protected from impacts at all costs.
All UK helmets now sold must comply with EC22/05 to be road legal.