Other than in certain circumstances which don't apply here, it's illegal to sell DVDs which haven't received a BBFC classification.
Pornographic DVDs, which meet the relevant criteria, are granted an R18 certificate. This means that they can only be sold in licensed sex shops. (The license is granted by the local authority). They may not be sold by mail order. So, unless you open a sex shop (and get the premises licensed), it's illegal to sell pornographic DVDs:
http://www.bbfc.co.uk/classification/c_R18.php
Chris