Without naming any specific companies, the things which really are a complete waste of time are:
1. Those wholesalers which insist on having ridiculous prices, and then applying some entirely spurious 'discount'. Such as a roll of cable priced £900, but you can have 95% off, which just makes it the same price as anywhere else.
These are always the ones who can't actually provide prices for anything, don't have price lists, and generally get invoices wrong on a regular basis. This is usually because some discount was 'forgotten' or the wrong code was used, or some other feeble excuse.
Companies operating like this deserve to go out of business. The days of 'list prices' and silly discounts ended decades ago.
2. Those which have printed catalogues / price lists, and have 'special deals' on the front pages, usually involving buying 100+ of some item such as ceiling roses. Unfortunately, these special deals usually end up being significantly more expensive than buying the exact same item in B&Q, and in B&Q you can buy one, not a whole box of 100 which could last for months/years.
3. Those who don't have the most basic and often used items in stock. Not just once, but on a regular basis
Seems certain wholesalers only order in a single roll of 2.5mmT+E, and when sold, it's 2 days before they get another one.