eu> Apple does not authorise sales through Mail Order.  As a result mail
  eu> order companies have to obtain their machines by the grey market.
  eu> 
  eu> This market is supplied with machines from authorised resellers who
  eu> have more machines than they can sell.  They come into this state of
  eu> affairs by overordering either accidentally or deliberatly to get a
  eu> better wholsale price from Apple.  In either case they often obscure
  eu> the serial nunber to protect their identity.  As a result the warranty
  eu> is void.
  
I have ordered several Macs from different mail order companies with absolutely zero problem. You have to dig around to find the true gray market dealers that sell Macs with authentic serial numbers untouched. There are value-added dealers (nothing to do with VAT, no flame please) that are very legitimate.

CDA unfortunately is one of those that replace the serial number with their own to prevent Apple from tracing which authorized dealer sold that machine to CDA.