People are selling single sim cards and these are only being obtained from the actual o2/orange/virgin ect sites becuase of promotions, these sites continuously run promos for free sim cards and are posted on forums and in emails.
Its one simple form... by all means buy them off ebay.
Its good buying bulk sims off ebay but single ones can get to you in just the same time from these promotions and you can get upto 4 free !!!! at one time..
Hope this helps.