What no-one has brought up is that trade sales are not covered by the usual consumer laws. That can mean there is no warranty at all on things you buy, and generally, means that if you buy excess stock, you get charged 20% or so to take the bits back, something that most consumers would not tolerate if they got 4 pairs of socks from M+S, then decided to take them back.
They are also there to deal quickly with people who know what they want, rather than explaining to Joe Public the parts they require, and what they look like etc.
Basically, Trade Counters are sparse, poor customer relations shops, which is fine, as the people using it know exactly what they require, and dont want to pay for a personal advisor’ to tell them which brick/pipe/cable to buy.
Alan.