Officially no they don't,hence the name TRADE Parts Service.It's up to the manager of each branch whether they do or don't sell to the public.
It comes down to the whim of the sales clerk. I was buying lots of coolant a year or so ago when i had a leak in the radiator. Most of te times I would pay around £6.50 for a 1.5L concentrate bottle of g14, but occasionally I would pay around £9.50 for it (retail price). It depends who you deal with and if they're feeling charitable they might push a trade discount through for you.As I understand it they all have trade counters and sell to the public. The issue is that normally they charge retail prices to the public in those situations and if you have a trade account you get the benefit of the discount which is quite a lot.
Fortunately I know a guy who has an account so I use his details and get the discount.