I'm confused as to how the website can 'tell you nothing'. It quite clearly states "Costco is an exclusive wholesale cash and carry membership warehouse club and is not open to the general public. Membership is required. You are eligible for trade membership if you are a bona fide business owner/manager who can provide evidence of trading".
It then goes on to explain the costs of membership, here:
http://www.costco.co.uk/Membership_home/Trade_ memship/trade_membership.htm
That page states that members can add up to six extra cards at a cost of �12 each per year. In practice, Costco probably follow the practice of other cash-&-carry outlets and make these cards available, for free, to employees of firms which take up membership. (e.g. schools often have some cards to be used by members of their staff. These are theoretically to be used only for purchases on behalf of the school but everyone, including the cash-&-carry company, knows that they'll actually be used for private purchases).
All cash-&-carry outlets have to at least pretend to only supply genuine traders. If they admitted to selling directly to the general public, they'd be breaching their agreements with manufacturers for the supply of goods. More importantly, most cash-&-carry warehouses don't have the relevant planning permission to sell direct to the public. They have to be able to assure the local authority that they only sell to traders, or risk enforced closure.
Chris