Online or offline, you still need to register as self-employed/ a business and pay all of the necessary taxes. They may not have a company registration number though, it all depends on their trading status.
Sounds to me like you've came across a supplier, who doesn't know the ropes. These are the kind of deals that often go wrong, look for a lengthy thread in this section from yesterday (my point exactly).
Further information on the topic can be found on the HMRC and Business Link websites. Definitely worth a look.